|Synthesis Strategies and Ferroelectricity of Noncentrosymmetric Perovskite Oxide Nanostructures|
Perovskite oxides are important class of materials possessing high dielectric and piezoelectric coefficients, switchable ferroelectric (FE) polarization, large nonlinear optical coefficients and interesting electrical properties. These properties may be exploited in applications such as nonvolatile memory devices, thermistors, multilayer capacitors, dynamic random access memories and as cathode electron sources, and photovoltaic applications. Noncentrosymmetric (NCS) derivatives of perovskite oxides are studied for their symmetry-dependent polarization properties . In particular, Pb-free LiNbO3 (LN)-type Zn-based oxides such as ZnSnO3 and ZnTiO3 with R3c space group have attracted special attention due to their theoretically predicted high spontaneous polarization (59 µC/cm2 for ZnSnO3, 88 μC/cm2 for ZnTiO3) [1,2,3]. Demonstration of large remanent polarization in epitaxial LN-type ZnSnO3 thin films (Pr ≈ 47 µC/cm2)  and recently shown evidence of high polarization in ZnSnO3 nanowire arrays (Pr ≈ 30 µC/cm2) by our group [5,6], have experimentally validated the theory. Synthesis and integration of NCS-type materials with ordered low-dimensional structures and controlled crystal orientations is a challenge, due to their complicated methodologies, high-cost and difficulties with phase stability. Particularly for Pb-free complex NCS oxides, it is difficult to synthesize these metastable phases by traditional solid state synthesis processes under ambient conditions. In this talk, I will discuss a non-traditional combined physical/chemical process encompassing pulsed-laser deposition (PLD) technique and a solvothermal synthesis to fabricate nanostructure arrays of phase pure LN-type perovskite oxides on a variety of substrates. The nanostructures of these useful perovskite oxide phases are stabilized through chemical synthesis of initially PLD deposited highly oriented nanoscale seed-layers of materials of alike symmetry with minimum lattice mismatch (ZnO, SnO2, TiO2). The as-grown nano- and micro-structured phases showed unique electrical and FE (hysteresis) properties which are hitherto unknown in these materials systems [5,6]. Furthermore, the effects of doping and surface modification by core-shell on the bandgap tunability in these novel perovskite materials will be shown. Our research indicated that bandgap is an extremely strong function of the lattice strain in ZnSnO3, which can be tuned to modulate properties at the nano-scale. The facile, low-cost method for fabricating high quality LN-type perovskite nanostructure arrays may expand the outreach of probes for understanding the structure-property relations in these technologically important materials for smart memories andsuggest their possible applications in next generation ferroelectric solar cells.
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|Nanomagnetism in closed-shell systems|
The magnetic properties of nano-materials can differ substantially from those of the same substance in bulk. This is particularly true for systems in closed-shell electronic states, which are diamagnetic in bulk, e.g. gold, but exhibit a very rich magnetic behavior in the nano-scale. In addition to this emergent behavior, which is determined essentially by the symmetry of spin occupation at the Fermi level of the material, the optical response of nano-particles may be strongly influenced by their magnetic properties and, in some cases, an inverse correlation between plasmon resonance absorption and the onset of magnetization has been observed.
In this talk I will review some of the most striking experimental evidence on anomagnetism as well as our recent theoretical work on the subject, including gold nanoparticles and atomically precise gold clusters, where the observed magnetic properties depend strongly on chemisorption due to changes in the electronic properties of the anchoring group. We consider both purely quantum models to explore the spin symmetry breaking ultimately responsible for the magnetic behavior in closed-shell systems as well as mean-field statistical models to interpret the size-dependent behavior of the magnetization.
- K. S. Krishna, P. Tarakeshwar, V. Mujica, and C. S. S. T. Kumar, "Chemically Induced Magnetism in Atomically Precise Gold Clusters", Small 10 (2014) 907.
- A. Roldán, F. Illas, P. Tarekeshwar and V. Mujica, "Stability and Quenching of Plasmon Resonances in Magnetic Gold Nanoparticles", J. Phys. Chem. Lett. 2 (2011) 2996.