Self-Assembled Magnetic Metallic Nanopillars in Ceramic Matrix with Anisotropic Magnetic and Electrical Transport Properties.

Ordered arrays of metallic nanopillars embedded in a ceramic matrix have recently attracted considerable interest for their multifunctionality in advanced devices. A number of hurdles need to be overcome for achieving practical devices, including selections of metal-ceramic combination, creation of tunable and ordered structure, and control of strain state. In this article, we demonstrate major advances to create such a fine nanoscale structure, i.e., epitaxial self-assembled vertically aligned metal-ceramic composite, in one-step growth using pulsed laser deposition. Tunable diameter and spacing of the nanopillars can be achieved by controlling the growth parameters such as deposition temperature. The magnetic metal-ceramic composite thin films demonstrate uniaxial anisotropic magnetic properties and enhanced coercivity compared to that of bulk metal. The system also presents unique anisotropic electrical transport properties under in-plane and out-of-plane directions. This work paves a new avenue to fabricate epitaxial metal-ceramic nanocomposites, which can simulate broader future explorations in nanocomposites with novel magnetic, optical, electrical, and catalytical properties.

Self-Assembled Epitaxial Au-Oxide Vertically Aligned Nanocomposites for Nanoscale Metamaterials.

Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques with limited paths toward reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned gold (Au) nanopillars (∼20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales.

Ionic Conductivity Increased by Two Orders of Magnitude in Micrometer-Thick Vertical Yttria-Stabilized ZrO2 Nanocomposite Films.

We design and create a unique cell geometry of templated micrometer-thick epitaxial nanocomposite films which contain ~20 nm diameter yttria-stabilized ZrO2 (YSZ) nanocolumns, strain coupled to a SrTiO3 matrix. The ionic conductivity of these nanocolumns is enhanced by over 2 orders of magnitude compared to plain YSZ films. Concomitant with the higher ionic conduction is the finding that the YSZ nanocolumns in the films have much higher crystallinity and orientation, compared to plain YSZ films. Hence, "oxygen migration highways" are formed in the desired out-of-plane direction. This improved structure is shown to originate from the epitaxial coupling of the YSZ nanocolumns to the SrTiO3 film matrix and from nucleation of the YSZ nanocolumns on an intermediate nanocomposite base layer of highly aligned Sm-doped CeO2 nanocolumns within the SrTiO3 matrix. This intermediate layer reduces the lattice mismatch between the YSZ nanocolumns and the substrate. Vertical ionic conduction values as high as 10(-2) Ω(-1) cm(-1) were demonstrated at 360 °C (300 °C lower than plain YSZ films), showing the strong practical potential of these nanostructured films for use in much lower operation temperature ionic devices.

Perpendicular Exchange-Biased Magnetotransport at the Vertical Heterointerfaces in La(0.7)Sr(0.3)MnO3:NiO Nanocomposites.

Heterointerfaces in manganite-based heterostructures in either layered or vertical geometry control their magnetotransport properties. Instead of using spin-polarized tunneling across the interface, a unique approach based on the magnetic exchange coupling along the vertical interface to control the magnetotransport properties has been demonstrated. By coupling ferromagnetic La0.7Sr0.3MnO3 and antiferromagnetic NiO in an epitaxial vertically aligned nanocomposite (VAN) architecture, a dynamic and reversible switch of the resistivity between two distinct exchange biased states has been achieved. This study explores the use of vertical interfacial exchange coupling to tailor magnetotransport properties, and demonstrates their viability for spintronic applications.

Strain and interface effects in a novel bismuth-based self-assembled supercell structure.

Bi2FeMnO6 (BFMO) thin films with both conventional pseudocubic structure and novel supercell structure have been grown on SrTiO3 (001) substrates with different thicknesses of CeO2 buffer layers (ranging from 6.7 to 50.0 nm) using pulsed laser deposition. The correlation between the thickness of the CeO2 buffer layer and the structure of the BFMO films shows that the CeO2 buffer layer, as thin as 6.7 nm, is sufficient in triggering the novel BFMO supercell structure. This may be ascribed to the interfacial strain between the BFMO supercell structure and the CeO2 buffer layer which also serves as a seed layer. The buffer layer thickness is found to be critical to control the microstructure and magnetism of the formed BFMO supercell structures. Thin seed layers can produce a smoother interface between the BFMO film and the CeO2 buffer layer, and therefore better ferrimagnetic properties. Our results have demonstrated that strain and interface could be utilized to generate novel thin film structures and to tune the functionalities of thin films.

Ferroelectric properties of vertically aligned nanostructured BaTiO3-CeO2 thin films and their integration on silicon.

Epitaxial (BaTiO3)0.5(CeO2)0.5 films have been deposited in vertically aligned nanocomposite form on SrTiO3/TiN buffered Si substrates to achieve high-quality ferroelectrics on Si. The thin TiN seed layer promotes the epitaxial growth of the SrTiO3 buffer on Si, which in turn is essential for the high-quality growth of the vertically aligned nanocomposite structure. X-ray diffraction and transmission electron microscopy characterization show that the films consist of distinct c-axis oriented BaTiO3 and CeO2 phases. Polarization measurements show that the BaTiO3-CeO2 films on Si are actually ferroelectric at room temperature, and the ferroelectric response is comparable to pure BaTiO3 as well as the BaTiO3-CeO2 films on SrTiO3 single-crystalline substrates. Capacitance-voltage measurements show that, instead of decreasing, the Curie temperature increases to 175 and 150 °C for the samples on SrTiO3 and Si substrates, respectively. This work is an essential step towards integrating novel nanostructured materials with advanced functionalities into Si-based devices.

Integration of self-assembled vertically aligned nanocomposite (La0.7Sr0.3MnO3)(1-x):(ZnO)x thin films on silicon substrates.

Epitaxial (La0.7Sr0.3MnO3)(1-x):(ZnO)x (LSMO:ZnO) in vertically aligned nanocomposite (VAN) form was integrated on STO/TiN-buffered silicon substrates by pulsed-laser deposition. Their magnetotransport properties have been investigated and are systematically tuned through controlling the ZnO concentration. The composite film with 70% ZnO molar ratio exhibits a maximum magnetoresistance (MR) value of 55% at 70 K and 1 T. The enhanced tunable low-field MR properties are attributed to structural and magnetic disorders and spin-polarized tunneling through the secondary ZnO phase. The integration of LSMO:ZnO VAN films on silicon substrates is a critical step enabling the application of VAN films in future spintronic devices.