Composition Graded, Epitaxial Oxide Nanostructures: Fabrication and Properties (NSF NIRT Grant # ) Efstathios I. Meletis 1, Jiechao Jiang 1, Chonglin Chen 2, Amar S. Bhalla 2, and Gemunu Gunaratne 3 1 University of Texas at Arlington, Arlington, Texas; 2 University of Texas at San Antonio, San Antonio, Texas; 3 University of Houston, Houston, Texas BACKGROUND U H Perovskite oxides are of enormous fundamental interest and technological importance due to their intriguing properties. These properties can be tailored for a wide range of applications in magnetic, magneto-electronic, photonic, and spintronic technology. Many perovskite - type oxides have been synthesized in the past in bulk form or as thin films. It is expected that nanostructures of these oxides may offer enormous opportunities to explore intriguing physics and applications. However, synthesis of one-dimensional perovskite -type oxides is a challenge and has seen very little success due to their complex composition. Recently, we achieved fabrication of self-organized, ordered arrays of coherent, orthogonal epitaxial (La, Sr)MnO 3 nanopillars on (001) LaAlO 3 by pulsed-laser deposition (PLD) [1], which to the best of our knowledge, has been the first report on the fabrication by self-organization of such epitaxial oxide nanopillars. The formation of the nanopillars depends strongly on the processing temperature and oxide composition [2]. Such nanopillars exhibit novel magnetic properties different from those of their bulk, thin film, or nanoparticle counterparts [3]. Furthermore, ferroelectric, compositionally gradient thin films have been shown to tremendously enhance piezoelectric response due to the build-in strain gradient. The coexistence of different properties that can be coupled in nanocomposite thin films has recently stimulated much scientific and technological interest since the coupling can provide new property tenability. However, major challenges exist in extending these compositional variations from thin films to nanopillars since the fabrication of compositionally graded and modulated composite nanopillars by self-organization has not yet been attempted.. OBJECTIVES Investigate the principles of formation of self-assembled, epitaxial nanopillars of ferromagnetic (La,Sr)MnO 3 and (La,Ca)MnO 3, and ferroelectric (Ba,Sr)TiO 3 and Ba(Ti,Zr)O 3 perovskite-oxides; Fabricate compositionally graded and modulated composite (La,Sr)MnO 3 and (La,Ca)MnO 3, and ferroelectric (Ba,Sr)TiO 3 and Ba(Ti,Zr)O 3 nanopillars; Characterize and study the mechanism of morphological evolution, structure and physical properties; Theoretically identify relationships between nanostructure characteristics and materials properties; Develop a tool for designing and exploring 1-D nanostructures of interest and other new materials. Project flow chart for interaction between team members and overall contribution to Design of new materials. The specifications and long term vision have been discussed with the project team members during the kickoff meeting (Oct. 1st, 2007, UTA). These specifications were updated during the 2nd (May 26, 2008, UTSA) and 3rd (Oct. 16, 2008, UH) project meeting according to the project feedback mechanism. (II) Double-layered Nanostructure of Ba(Zr,Ti)O 3 Epilayer and twin–coupled domain structures on MgO substrate (III) Two-dimensional Interfacial Structure of the Epitaxial Oxide Films on MgO Fig. 9 Ordered quantum- dot array. Knowledge obtained from these series of investigations is used for theoretical modeling for further precisely controlling the formation of the nanopillar structures. Structures formed during the growth of an epilayer on a substrate are determined by minimizing the energy of the configuration, which consists of (1) elastic energy of the epilayer, due to the requirement that it be commensurate with the substrate, (2) the surface energy of the epilayer, and (3) the wetting potential [6]. We assume that the substrate lattice is unchanged, and hence that there is no associated energy. The spatio-temporal dynamics of the epilayer is typically described using the evolution of its height h(x,y) via (IV) Theory and Modeling of Self-assembling of Nanostructured Films In deriving this equation, we have scaled h(x,y) by the height L at which the homogeneous layer destabilizes. The control parameters L, g, p, and q can be evaluated n terms of the mechanical parameters of the substrate and the epilayer. Under the model dynamics, a uniform (but noisy) deposition of atoms on a substrate gives self- assembled quantum-dot arrays. In order to form large-scale perfect arrays, we use a technique that we proposed previously; namely masking of the deposition [3]. Properties of the mask can be determined from the spatio-temporal dynamics of the formation of a disordered pattern in the absence of the mask [8]. where the diffusion is along the surface h(x,y) [1]. Unfortunately, the expressions for the terms on the right (the free energy density, curvature, wetting energy etc.) in terms of h(x,y) and its derivatives are very complicated. The analysis can be simplified by using the “small slope” expansion, which will be valid close to the Stransky-Krastonow instability, where the homogeneous solution destabilizes to a patterned array [7]. Under these conditions, the previous equation reduces to References: [1] J.C. Jiang, E.I. Meletis and K.I. Gnanasekar, “Self-organized, ordered array of coherent orthogonal column nanostructures in epitaxial La 0.8 Sr 0.2 MnO 3 thin films”, Appl. Phys. Lett. vol 80, , [2] J.C. Jiang, K.I. Gnanasekar and E.I. Meletis, “Composition and Growth Temperature Effect on the Microstructure of Epitaxial La 1-x Sr x MnO 3 Thin Films on (100) LaAlO 3 ”, J. Mater. Res., vol. 18, , [3] J.C. Jiang, L.L. Henry, K.I. Gnanasekar, C.L. Chen and E.I. Meletis, “Self-Assembly of Highly Epitaxial (La,Sr)MnO 3 Nanorods on (001) LaAlO 3 ”, Nano Letters, vol. 14, , [4] J.C. Jiang, Z. Yuan, C.L. Chen and E.I. Meletis, “Interface Modulated Structure of Highly Epitaxial (Pb,Sr)TiO3 Thin Films on (001) MgO” Appl. Phys. Lett., vol. 90, Art. No (2007). [5] J. C. Jiang, J. He, E.I. Meletis, J. Liu, Z. Yuan, and C. L. Chen, “Two-dimensional Modulated Interfacial Structures of Highly Epitaxial Ferromagnetic (La,Ca)MnO 3 and Ferroelectric (Pb,Sr)TiO 3 Thin Films on (001) MgO” Journal of Nano Research, vol. 3, (2008). [6] B. J. Spencer, P. W. Voohees, and S. H. Davis, “Morphological Instability in Epitaxially Strained Dislocation-Free Solid Films: Linear Stability Theory,” J. Appl. Phys. 73, 4955 (1993). [7] A. A. Golovin, M. S. Levine, T. V. Savina, and S. H. Davis, “Faceting Instability in the Presence of Wetting Interactions: A Mechanism for the Formation of Quantum Dots,” Phys. Rev. B 70, (2004). [8] F. Shi, P. Sharma, D J. Kouri, F. Hussain, and G. H. Gunaratne, “Nanostructures with Long-Range Order in Monolayer Self-Assembly,” Phys. Rev. E 78, (2008). (I) Epitaxial (La,Sr)MnO 3 Layer and Nanopillar Structures The nature of the interfacial structure is very important in understanding the growth mechanism of epitaxial films and nanopillars. Cross-section TEM has been widely used to study the interfacial structure of heteroepitaxial films and has been turned out to be a very effective technique for such studies. The lattice misfit induced strain energy can be partially or fully released at the interface between the epitaxial film and substrate by edge dislocation formation which can be periodically distributed along the interface. However, the interfacial structure information obtained using cross-section TEM is limited in one- dimensional space. More local information is needed in order to completely understand the influence of the substrate surface characteristics and film/substrate interface on the microstructure of epitaxial films. As a part of this project, we recently developed a method using plan-view TEM to study the interface structure in 2D space, which is able to provide critical and valuable information that is lacking from the cross-section TEM analysis [4]. We have fabricated and studied epitaxial (La,Ca)MnO 3 and (Pb,Sr)TiO 3 films on MgO substrate. The lattice mismatch near the interface regions obtained using the new method was found to be -8.0% for (La,Ca)MnO 3 /MgO and -7.14% for PbTiO 3 /MgO. Both values are larger than those obtained using cross-section TEM (-6.4% for (La,Ca)MnO 3 /MgO and -6.2 % for PbTiO 3 /MgO). The (Pb,Sr)TiO 3 film is well commensurate with the substrate over large areas, whereas (La,Ca)MnO 3 film is only locally commensurate with the substrate [5]. We have systematically investigated the effects of temperature, pressure, laser energy and frequency and post-annealing on the microstructure formation of epitaxial (La,Sr)MnO 3 thin films. We are able to fabricate (La,Sr)MnO 3 continuous epilayer (Fig. 1a) and discrete epitaxial nanopillars (Figs. 1b and c) by manipulating the experimental conditions and parameters and confirmed the repeatability for achieving a variety of designed nanostructures. A roadmap for fabricating various distinct epitaxial nanostructures has been established. Figure 1. XTEM image of epitaxial (La,Sr)MnO 3 continuous film (a) and nanopillars (b) on (001) LaAlO 3 substrate. (c) Plan-view TEM of epitaxial (La,Sr)MnO 3 nanopillars. We recently identified non-lead ferroelectric material, BaTiO 3 and its modified materials (Ba, Sr)TiO 3 and Ba(Zr,Ti)O 3 as another counterpart for the composition graded nanostructures. These films exhibit high dielectric constant, low dielectric loss tangent and large electric field tunability that have attracted considerable attention for bypass capacitors, IR detectors, and tunable microwave applications. We fabricated “structure” graded Ba(Zr,Ti)O 3 films on (001) MgO. Ba(Zr,Ti)O 3 epilayer was first epitaxially grown on the substrate followed by a layer of multi-oriented twin domain structures by sharing their {111} planes with the epilayer. Such structure graded thin films show interesting abnormal ferroelectric properties that do not exist in their bulk counterpart. Fig. 2 XTEM image of twin-coupled structure on epitaxial Ba(Zr,Ti)O 3 film on (001) MgO. (a) Bright-field image, (b), (c) and (d) dark-field images showing presence of BZT epilayer and two twins. Fig. 3 Schematic illustration of the epilayer structure and the four possible oriented twin domains (up) and their crystallographic orientation relationships between the epilayer and the twin domains (down). Black blocks (squares, triangles and ellipses) represent the zone axes of the epilayer, while those filled with coarse slope, horizontal, fine slope and vertical lines represent the zone axes of Twin-1, Twin- 2, Twin-3 and Twin-4, respectively. Fig. 4 Hysteresis loop measurement of BZT film exhibiting interesting abnormal properties due to the formation of the twins. Fig. 5 Plan-view TEM (a) bright-field and dark-field images (b), (c), (d) and (e) showing presence of twin domain Twin-1 (T1), Twin-2 (T2), Twin-3 (T3), and Twin- 4 (T4), respectively. Epilayer Twins Fig. 6 HRTEM image of a plan- view TEM sample showing coexistence of epilayer and twins. Fig. 7 Cross-section TEM (a) bright-field image and (b) EDP of the PSTO/MgO interface; (c) bright-field image and (d) EDP of the LCMO/MgO interface. Fig. 8 (a) EDP and (b) HRTEM of plan-view PSTO/MgO interface; (c) EDP and (c) HRTEM of plan-view LCMO/MgO interface. Educational Outreach UTA established a working relationship with the Society of Hispanic Professional Engineers (SHPE) sponsoring six (6) Hispanic students to participate in Pre-college Symposia and developing a Latino Summer Camp on UTA campus during the Summer of Fig Latino Summer Camp on UTA campus: demonstration of temperature effects on the mechanical behavior of engineering materials.