Review on the growth, properties and applications of self-assembled oxide-metal vertically aligned nanocomposite thin films-current and future perspectives.

Self-assembled oxide-metal nanocomposite thin films have aroused great research interest owing to their wide range of functionalities, including metamaterials with plasmonic and hyperbolic optical properties, and ferromagnetic, ferroelectric and multiferroic behaviors. Oxide-metal nanocomposites typically self-assemble as metal particles in an oxide matrix or as a vertically aligned nanocomposite (VAN) with metal nanopillars embedded in an oxide matrix. Among them, the VAN architecture is particularly interesting due to the vertical strain control and highly anisotropic structure, enabling the epitaxial growth of materials with large lattice mismatch. In this review, the driving forces behind the formation of self-assembled oxide-metal VAN structures are discussed. Specifically, an updated in-plane strain compensation model based on the areal strain compensation concept has been proposed in this review, inspired by the prior linear strain compensation model. It provides a guideline for material selection for designing VAN systems, especially those involving complex orientation matching relationships. Based on the model, several case studies are discussed, comparing the microstructure and morphology of different oxide-metal nanocomposites by varying the oxide phase. Specific examples highlighting the coupling between the electrical, magnetic and optical properties are also discussed in the context of oxide-metal nanocomposites. Future research directions and needs are also discussed.

Self-Assembled BaTiO3-AuxAg1-x Low-Loss Hybrid Plasmonic Metamaterials with an Ordered "Nano-Domino-like" Microstructure.

Metallic plasmonic hybrid nanostructures have attracted enormous research interest due to the combined physical properties coming from different material components and the broad range of applications in nanophotonic and electronic devices. However, the high loss and narrow range of property tunability of the metallic hybrid materials have limited their practical applications. Here, a metallic alloy-based self-assembled plasmonic hybrid nanostructure, i.e., a BaTiO3-AuxAg1-x (BTO) vertically aligned nanocomposite, has been integrated by a templated growth method for low-loss plasmonic systems. Comprehensive microstructural characterizations including high-resolution scanning transmission electron microscopy (HRSTEM), energy-dispersive X-ray spectroscopy (EDS), and three-dimensional (3D) electron tomography demonstrate the formation of an ordered "nano-domino-like" morphology with Au0.4Ag0.6 nanopillars as cylindrical cores and BTO as square shells. By comparing with the BTO-Au hybrid thin film, the BTO-Au0.4Ag0.6 alloyed film exhibits much broader plasmon resonance, hyperbolic dispersion, low-loss, and thermally robust features in the UV-vis-NIR wavelength region. This study provides a feasible platform for a complex alloyed plasmonic hybrid material design with low-loss and highly tunable optical properties toward all-optical integrated devices.

Thermal stability of self-assembled ordered three-phase Au-BaTiO3-ZnO nanocomposite thin films via in situ heating in TEM.

Thermal stability of oxide-metal nanocomposites is important for designing practical devices for high temperature applications. Here, we study the thermal stability of the self-assembled ordered three-phase Au-BaTiO3-ZnO nanocomposite by both ex situ annealing under air and vacuum conditions, and by in situ heating in TEM in a vacuum. The study reveals that the variation of the annealing conditions greatly affects the resulting microstructure and the associated dominant diffusion mechanism. Specifically, Au nanoparticles show coarsening upon air annealing, while Au and Zn either form a solid solution, with Zn atomic percentage less than 10%, or undergo a reverse Vapor-Liquid-Solid (VLS) mechanism upon vacuum annealing. The distinct microstructures obtained also show different permittivity response in the visible and near-infrared region, while retaining their hyperbolic dispersion characteristics enabled by their highly anisotropic structures. Such in situ heating study in TEM provides critical information about microstructure evolution, growth mechanisms at the nanoscale, and thermal stability of the multi-phase nanocomposites for future electronic device applications.

Au-Encapsulated Fe Nanorods in Oxide Matrix with Tunable Magneto-Optic Coupling Properties.

Materials with magneto-optic coupling properties are highly coveted for their potential applications ranging from spintronics and optical switches to sensors. In this work, a new, three-phase Au-Fe-La0.5Sr0.5FeO3 (LSFO) hybrid material grown in a vertically aligned nanocomposite (VAN) form has been demonstrated. This three-phase hybrid material combines the strong ferromagnetic properties of Fe and the strong plasmonic properties of Au and the dielectric nature of the LSFO matrix. More interestingly, the immiscible Au and Fe phases form Au-encapsulated Fe nanopillars, embedded in the LSFO matrix. Multifunctionalities including anisotropic optical dielectric properties, plasmonic properties, magnetic anisotropy, and room-temperature magneto-optic Kerr effect coupling are demonstrated. The single-step growth method to grow the immiscible two-metal nanostructures (i.e., Au and Fe) in the complex hybrid material form opens exciting new potential opportunities for future three-phase VAN systems with more versatile metal selections.

Bidirectional tuning of phase transition properties in Pt : VO2 nanocomposite thin films.

A phase transition material, VO2, with a semiconductor-to-metal transition (SMT) near 341 K (68 °C) has attracted significant research interest because of drastic changes in its electrical resistivity and optical dielectric properties. To address its application needs at specific temperatures, tunable SMT temperatures are highly desired. In this work, effective transition temperature (Tc) tuning of VO2 has been demonstrated via a novel Pt : VO2 nanocomposite design, i.e., uniform Pt nanoparticles (NPs) embedded in the VO2 matrix. Interestingly, a bidirectional tuning has been achieved, i.e., the transition temperature can be systematically tuned to as low as 329.16 K or as high as 360.74 K, with the average diameter of Pt NPs increasing from 1.56 to 4.26 nm. Optical properties, including transmittance (T%) and dielectric permittivity (ε') were all effectively tuned accordingly. All Pt : VO2 nanocomposite thin films maintain reasonable SMT properties, i.e. sharp phase transition and narrow width of thermal hysteresis. The bidirectional Tc tuning is attributed to two factors: the reconstruction of the band structure at the Pt : VO2 interface and the change of the Pt : VO2 phase boundary density. This demonstration sheds light on phase transition tuning of VO2 at both room temperature and high temperature, which provides a promising approach for VO2-based novel electronics and photonics operating under specific temperatures.

Tunable physical properties in BiAl1-x Mn x O3 thin films with novel layered supercell structures.

Morphological control in oxide nanocomposites presents enormous opportunities for tailoring the physical properties. Here, we demonstrate the strong tunability of the magnetic and optical properties of Bi-based layered supercell (LSC) multiferroic structures, i.e., BiAl1-x Mn x O3, by varying the Al : Mn molar ratio. The microstructure of the LSC structure evolves from a supercell structure to Al-rich pillars in the supercell structure as the Al molar ratio increases. The LSC structures present excellent multiferroic properties with preferred in-plane magnetic anisotropy, a tunable band gap and anisotropic dielectric permittivity, all attributed to the microstructure evolution and their anisotropic microstructure. Three different strain relaxation mechanisms are identified that are active during thin film growth. This study provides opportunities for microstructure and physical property tuning which can also be explored in other Bi-based LSC materials with tailorable multiferroic and optical properties.

Breaking Lattice Symmetry in Highly Strained Epitaxial VO2 Films on Faceted Nanosurface.

The lattice symmetry of strongly correlated oxide heterostructures determines their exotic physical properties by coupling the degrees of freedom between lattices and electrons, orbitals, and spin states. Systematic studies on VO2, a Mott insulator, have previously revealed that lattice distortion can be manipulated by the interfacial strain and electronic phase separation can emerge. However, typical epitaxial film-substrate interface strain provides a very limited range for exploring such interface-engineered phenomena. Herein, epitaxially grown VO2 thin films on asymmetrically faceted m-plane sapphire substrates with the hill-and-valley type surfaces have been demonstrated. Interestingly, lattice symmetry breaking has been proven based on the large residual strain from the different faceted planes. By this lattice symmetry breaking, electronic phase separation and metal-insulator transition in the VO2 films are modulated, and anisotropy in optical responses is exhibited. These results on asymmetrical interfacial engineering in oxide heterostructures open up new routes for novel functional materials design and functional electro/optic device nanofabrication.

Tuning magnetic anisotropy in Co-BaZrO3 vertically aligned nanocomposites for memory device integration.

Ferromagnetic nanostructures with strong anisotropic properties are highly desired for their potential integration into spintronic devices. Several anisotropic candidates, such as CoFeB and Fe-Pt, have been previously proposed, but many of them have limitations such as patterning issues or thickness restrictions. In this work, Co-BaZrO3 (Co-BZO) vertically aligned nanocomposite (VAN) films with tunable magnetic anisotropy and coercive field strength have been demonstrated to address this need. Such tunable magnetic properties are achieved through tuning the thickness of the Co-BZO VAN structures and the aspect ratio of the Co nanostructures, which can be easily integrated into spintronic devices. As a demonstration, we have integrated the Co-BZO VAN nanostructure into tunnel junction devices, which demonstrated resistive switching alluding to Co-BZO's immense potential for future spintronic devices.

Self-Assembled Ordered Three-Phase Au-BaTiO3 -ZnO Vertically Aligned Nanocomposites Achieved by a Templating Method.

Complex multiphase nanocomposite designs present enormous opportunities for developing next-generation integrated photonic and electronic devices. Here, a unique three-phase nanostructure combining a ferroelectric BaTiO3 , a wide-bandgap semiconductor of ZnO, and a plasmonic metal of Au toward multifunctionalities is demonstrated. By a novel two-step templated growth, a highly ordered Au-BaTiO3 -ZnO nanocomposite in a unique "nanoman"-like form, i.e., self-assembled ZnO nanopillars and Au nanopillars in a BaTiO3 matrix, is realized, and is very different from the random three-phase ones with randomly arranged Au nanoparticles and ZnO nanopillars in the BaTiO3 matrix. The ordered three-phase "nanoman"-like structure provides unique functionalities such as obvious hyperbolic dispersion in the visible and near-infrared regime enabled by the highly anisotropic nanostructures compared to other random structures. Such a self-assembled and ordered three-phase nanocomposite is obtained through a combination of vapor-liquid-solid (VLS) and two-phase epitaxy growth mechanisms. The study opens up new possibilities in the design, growth, and application of multiphase structures and provides a new approach to engineer the ordering of complex nanocomposite systems with unprecedented control over electron-light-matter interactions at the nanoscale.

Tailorable Au Nanoparticles Embedded in Epitaxial TiO2 Thin Films for Tunable Optical Properties.

The unique property of plasmonic materials to localize light into deep sub-wavelength regime has greatly driven various applications in the field of photovoltaics, sensors, and photocatalysis. Here, we demonstrate the one-step growth of an oxide-metal hybrid thin film incorporating well-dispersed gold (Au) nanoparticles (NPs) with tailorable particle shape and diameters (ranging from 2 to 20 nm) embedded in highly epitaxial TiO2 matrix, deposited using pulsed laser deposition. Incorporation of Au NPs reduces the band gap of TiO2 and enhances light absorption in the visible regime owing to the excitation of localized surface plasmons. Optical properties, including the plasmonic response and permittivity, and photocatalytic activities of the Au-TiO2 hybrid materials are effectively tuned as a function of the Au NP sizes. Such optical property tuning is well captured using full-field simulations and the effective medium theory for better understanding of the physical phenomena. The tailorable shape and size of Au NPs embedded in TiO2 matrix present a novel oxide-metal hybrid material platform for optical property tuning and highly efficient plasmonic properties for future oxide-based photocatalytic sensors and devices.

Microstructure, Magnetic, and Magnetoresistance Properties of La0.7Sr0.3MnO3:CuO Nanocomposite Thin Films.

(La0.7Sr0.3MnO3)0.67:(CuO)0.33 (LSMO:CuO) nanocomposite thin films were deposited on SrTiO3 (001), LaAlO3 (001), and MgO (001) substrates by pulsed laser deposition, and their microstructure as well as magnetic and magnetoresistance properties were investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that LSMO:CuO films grow as highly textured self-assembled vertically aligned nanocomposite (VAN), with a systematic domain structure and strain tuning effect based on the substrate type and laser deposition frequency. A record high low-field magnetoresistance (LFMR) value of ∼80% has been achieved in LSMO:CuO grown on LaAlO3 (001) substrate under high frequency. Detailed analysis indicates that both the strain state and the phase boundary effect play a significant role in governing the overall LFMR behavior.

Novel Layered Supercell Structure from Bi2AlMnO6 for Multifunctionalities.

Layered materials, e.g., graphene and transition metal (di)chalcogenides, holding great promises in nanoscale device applications have been extensively studied in fundamental chemistry, solid state physics and materials research areas. In parallel, layered oxides (e.g., Aurivillius and Ruddlesden-Popper phases) present an attractive class of materials both because of their rich physics behind and potential device applications. In this work, we report a novel layered oxide material with self-assembled layered supercell structure consisting of two mismatch-layered sublattices of [Bi3O3+δ] and [MO2]1.84 (M = Al/Mn, simply named BAMO), i.e., alternative layered stacking of two mutually incommensurate sublattices made of a three-layer-thick Bi-O slab and a one-layer-thick Al/Mn-O octahedra slab in the out-of-plane direction. Strong room-temperature ferromagnetic and piezoelectric responses as well as anisotropic optical property have been demonstrated with great potentials in various device applications. The realization of the novel BAMO layered supercell structure in this work has paved an avenue toward exploring and designing new materials with multifunctionalities.

Valley Polarization by Spin Injection in a Light-Emitting van der Waals Heterojunction.

The band structure of transition metal dichalcogenides (TMDCs) with valence band edges at different locations in the momentum space could be harnessed to build devices that operate relying on the valley degree of freedom. To realize such valleytronic devices, it is necessary to control and manipulate the charge density in these valleys, resulting in valley polarization. While this has been demonstrated using optical excitation, generation of valley polarization in electronic devices without optical excitation remains difficult. Here, we demonstrate spin injection from a ferromagnetic electrode into a heterojunction based on monolayers of WSe2 and MoS2 and lateral transport of spin-polarized holes within the WSe2 layer. The resulting valley polarization leads to circularly polarized light emission that can be tuned using an external magnetic field. This demonstration of spin injection and magnetoelectronic control over valley polarization provides a new opportunity for realizing combined spin and valleytronic devices based on spin-valley locking in semiconducting TMDCs.