Prospective Articles Self-assembled vertical heteroepitaxial nanostructures: from growth to functionalities Heng-Jui Liu, Wen-I Liang, and Ying-Hao Chu, Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Haimei Zheng, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Ramamoorthy Ramesh, Department of Materials Science and Engineering, University of California, Berkeley, California 94720 Address all correspondence to Ying-Hao Chu at yhc@cc.nctu.edu.tw (Received 24 December 2013; accepted 17 April 2014) Abstract Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mech- anism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to con- struct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contri- butions ranging from scientific value to practical applications in the foreseeable future. Introduction In the past decades, self-assembled nanocomposites have become a glamorous research topic because various kinds of fascinating and novel properties and multifunctionalities can be designed via the intercoupling among spin, orbital, charge, and lattice degrees of freedom within the materials. [1–3] Notably in the research field of complex oxides, the applicable functionalities for combination cover almost all application spectra, ranging from metallic, semiconducting, insulating, ferromagnetic, ferroelectric, multiferroic, superconducting, nonlinear optical effects, etc. [4] In addition, an important factor that affects the functionalities of composites is the connectivity, defined as the number of dimensions in which the components are self-connected. The possible connectivities can be sorted out in 16 different types: 0–0, 1–0, 2–0, 3–0, 1–1, 2–1, 3–1, 2–2, 3–2, 3–3, 0–1, 0–2, 0–3, 1–2, 1–3, and 2–3. However, the most common schemes, 0–3-type particulate composites, 2–2-type laminate composites, and 1–3-type fiber composites, are able to give rise to the specific anisotropic properties. [2,5] With the recent advancements in thin-film processes, the self- assembled composites can also be fabricated at nanoscale in a film-on-substrate geometry. Comparing with 2–2-type multi- layer films, 1–3-type nanocomposite thin films draw more attention because of the high interface-to-volume ratio and intriguing physical properties. The first successful demons- tration of such nanocomposite is the (La 0.67 Ca 0.33 MnO 3 ) 1-x – (MgO) x composite system on a (100)-oriented MgO substrate by means of the metal organic aerosol deposition method, by Lebedev et al. in 2002. [6] When the concentration of MgO exceeds 0.3 (x > 0.3), La 0.67 Ca 0.33 MnO 3 grains are separated and surrounded by the MgO matrix. A latter work by Moshnyaga et al. [7] also confirmed that the structural and mag- netotransport properties of the La 0.67 Ca 0.33 MnO 3 nanoclusters can be tuned by the tensile stress originating from the MgO second phase. Nevertheless, one of the most studied systems, recently, is the nanocomposite thin films consisting of perovs- kite and spinel materials, starting from the pioneering work of spinel CoFe 2 O 4 nanopillars embedded in perovskite BaTiO 3 grown on the (001)-oriented SrTiO 3 substrate. [8–10] This system exhibits an enhanced and controllable magnetoelectric (ME) coupling, wherein the magnetic property of the nanocomposites can be manipulated by applying an electric field and vice versa. In addition, controllability of intriguing functionalities, such as the enhanced low-field magnetoresistance (LFMR) in (La 0.7 Sr 0.3 MnO 3 ) 0.5 –(ZnO) 0.5 vertically aligned nanocompo- site (VAN) thin films, [11] the improved dielectric response in (BiFeO 3 ) 0.5 –(Sm 2 O 3 ) [12] 0.5 and the enhanced ferroelectricity in (BaTiO 3 ) 0.5 - (Sm 2 O 3 ) [13] 0.5 self-assembled vertical heteroepi- taxial nanostructures (VHNs), are continuously unveiled. In this review, we emphasize on the development of 1–3-type nanocomposite thin films, or the so-called “self-assembled VHN thin films”, in particular, the systems composed of perovskites and spinels. In the following section, we will first introduce the fabrication of the self-assembled VHN films by the pulsed laser deposition (PLD) process and the cor- responding growth mechanisms, which are extensively studied MRS Communications (2014), 4, 31–44 © Materials Research Society, 2014 doi:10.1557/mrc.2014.13 MRS COMMUNICATIONS • VOLUME 4 • ISSUE 2 • www.mrs.org/mrc ▪ 31 https://doi.org/10.1557/mrc.2014.13 Downloaded from https://www.cambridge.org/core. Lawrence Berkeley Nat'l Lab, on 07 Feb 2021 at 07:39:16, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.