Transition metal oxides serve as an excellent platform in studying novel physical properties in solids. Due to the strong correlation, the various degrees of freedom such as charge, lattice, spin, and orbital usually couple with each other, thereby exhibiting unexpected, yet often technologically relevant physical properties. Recent advances in fabricating high-quality complex oxide heterostructures has further expanded the possibility of discovering many fascinating phenomena and has offered opportunities for controlling the behavior through deliberate control of structure, chemical composition, and orientation at the atomic scale. Based on both cutting-edge fabrication techniques and sophisticated characterization methods, our goal is to participate in establishing an exciting field of functional oxides as a new era of materials science and unprecedented device applications. In order to accomplish this goal, we propose the following scheme of future research.

1. Advanced pulsed laser epitaxy and understanding thin film growth kinetics

Figure 1. Colored scanning transmission electron microscope image of LaCoO₃ site-specifically alloyed into Bi₄Ti₃O₁₂ film [2].

Fabrication of atomically controlled high quality samples is the most important first step for successful experimental materials science research. Understanding the detailed growth mechanism is itself an important field and is the key to reliable and reproducible samples. Pulsed laser epitaxy (PLE) is a versatile tool of fabricating single crystalline oxide heterostructures. In particular, it can be readily combined with various real-time growth characterization tools such as reflection high-energy electron diffraction (RHEED) or ion probes, in studying growth kinetics [1]. Good understanding of the growth kinetics leads to a successful fabrication of various oxide heterostructures for numerous scientific and application purposes, e.g., site-specific substitution for band gap tuning (Fig. 1) and fractional delta-doped oxide digital superlattices for studying two-dimensional metallic properties [2-4]. In addition to the already established growth characterization tools, we intend to further emply real-time spectroscopic tools for a more advanced and fundamental understanding on the formation of surfaces and interfaces.

2. Functional oxide interface and coupling of multiple order parameters across the interface

Figure 2. Schematic demonstration of interface engineering in perovskite transition metal oxides and coupling of charge, spin, orbital, and lattice degrees of freedom across the interface [1].

As in conventional semiconductors, the interface between transition metal oxides deserves intense scientific attention, and is relevant for many technological applications. More interestingly, oxide interfaces sometimes reveal utterly unexpected physical behaviors, different from their bulk constituents. As a prominent example, two-dimensional electron gas can be formed at the interface between two insulating oxides [1,4,5]. Applying ideas such as atomic layer engineering [1] or strain engineering [6] provides an opportunity to further control the interface and its physical properties. The expertise obtained from the first topic is crucial in studying the oxide interface, as the structural quality of the interface and its influence on different physical properties can be systematically studied. Central topics in transition metal oxides, such as metal-insulator transition, orbital correlations, spin-orbit coupling, multiferroicity, and superconductivity can be investigated with an emphasis on the interface physics [4,7,8]. Furthermore, this study can be extended to explore the multiple degrees of freedom within transition metal oxides, and their communications across the interface (Fig. 2).

3. Novel physics for energy and environmental application

Figure 3. Complex oxides for emerging energy applications [2].

Mainly due to the simplicity of the governing physics, physics has not been playing a major role in recent energy-related materials researches. However, as the importance of the sustainable energy is ever more increasing, we believe that a fundamental breakthrough mechanism or novel energy material should be proposed, based on the viewpoint of physics. We intend to investigate oxide heterostructures in terms of (1) solar energy application and (2) cathode materials for batteries and solid oxide fuel cells. In particular, we intend to continue our efforts in studying ferroelectric photovoltaics or optical band gap tuning in transition metal oxides (Fig. 3) [2,3]. On the other hand, we will also study cathode materials for battery and fuel cell applications, with an emphasis on multivalency of transition metal elements. Transition metal oxides are already one of the best materials for the cathode applications, as the valence state of the transition metal elements can be easily modified by introducing or extracting oxygen ions [9]. Based on our recent studies in multivalent cobalt oxides [6,10], we aim at understanding the valence state of different transition metals in complex oxides and their influence on energy and environment related performances.

References

[1] W. S. Choi, C. M. Rouleau, S. S. A. Seo, Z. Luo, H. Zhou, T. T. Fister, J. A. Eastman, P. H. Fuoss, D. D. Fong, J. Z. Tischler, G. Eres, M. F. Chisholm, and H. N. Lee “Atomic layer engineering of perovskite oxides for chemically sharp heterointerfaces” Adv. Mater. 24, 6423 (2012).

[2] W. S. Choi, M. F. Chisholm, D. J. Singh, T. Choi, G. E. Jellison Jr. and H. N. Lee, “Wide bandgap tunability in complex transition metal oxides by site-specific substitution” Nat. Commun. 3, 689 (2012).

[3] W. S. Choi and H. N. Lee “Band gap tuning in ferroelectric Bi₄Ti₃O₁₂ by alloying with LaTMO₃ (TM = Ti, V, Cr, Mn, Co, Ni, and Al)” Appl. Phys. Lett. 100, 132903 (2012).

[4] W. S. Choi, S. Lee, V. R. Cooper, and H. N. Lee “Fractionally delta-doped oxide superlattices for higher carrier mobilities” Nano Lett. 12, 4590 (2012).

[5] A. Ohtomo & H. Y. Hwang, “A high-mobility electron gas at LaAlO₃/SrTiO₃” Nature 427, 423 (2004).

[6] W. S. Choi, J.-H. Kwon, H. Jeen, J. E. Hamann-Borrero, A. Radi, S. Macke, R. Sutarto, F. He, G. A. Sawatzky, V. Hinkov, M. Kim, and H. N. Lee “Strain-induced spin-states in atomically ordered cobaltites” Nano Lett. 12, 4966 (2012).

[7] W. S. Choi, D. W. Jeong, S. S. A. Seo, Y. S. Lee, T. H. Kim, S. Y. Jang, H. N. Lee, and K. Myung-Whun “Charge states and magnetic ordering in LaMnO₃/SrTiO₃ superlattices” Phys. Rev. B 83, 195113 (2011).

[8] S. J. Moon, H. Jin, K. W. Kim, W. S. Choi, Y. S. Lee, J. Yu, G. Cao, A. Sumi, H. Funakubo, C. Bernhard, and T. W. Noh “Dimensionality-Controlled Insulator-Metal Transition and Correlated Metallic State in 5d Transition Metal Oxides Sr_n+1Ir_nO_3n+1 (n=1, 2, and ∞)” Phys. Rev. Lett. 101, 226402 (2008).

[9] M. Hibino, T. Kimura, Y. Suga, T. Kudo, and N. Misuno “Oxygen rocking aqueous batteries utilizing reversible topotactic oxygen insertion/extraction in iron-based perovskite oxides Ca_1-xLa_xFeO_3-δ” Sci. Rep. 2, 601 (2012).

[10] H. Jeen, W. S. Choi, M. D. Biegalski, C. M. Folkman, I. C. Tung, D. D. Fong, J. W. Freeland, D. Shin, H. Ohta, M. F. Chisholm, and H. N. Lee “Epitaxial oxygen sponges: Reversible redox reaction in epitaxial strontium cobalitites” under review (2013).