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Abstract Nanocomposite thin films, comprising two or more distinct materials at nanoscale, have attracted significant research interest considering their potential of integrating multiple functionalities for advanced applications in electronics, energy storage, photonics, photovoltaics, and sensing. Among various fabrication technologies, a one-step pulsed laser deposition process enables the self-assembly of materials into vertically aligned nanocomposites (VANs). The demonstrated VAN systems include oxide–oxide, oxide–metal, and nitride–metal VAN films and their growth mechanisms are vastly different. These complexities pose challenges in the designs, materials selection, and prediction of the resulted VAN morphologies and properties. The review examines the key roles that surface energy plays in the VAN growth and provides a generalized materials design guideline combining the two key factors of surface energy and lattice strain/mismatch, along with other factors related to growth kinetics that collectively influence the morphology of VAN films. This review aims to offer valuable guidelines for future material selection and microstructure design in the development of self-assembled VAN films. 

Abstract Complex oxide thin films cover a range of physical properties and multifunctionalities that are critical for logic, memory, and optical devices. Typically, the high‐quality epitaxial growth of these complex oxide thin films requires single crystalline oxide substrates such as SrTiO₃(STO), MgO, LaAlO₃, a‐Al₂O_3,and many others. Recent successes in transferring these complex oxides as free‐standing films not only offer great opportunities in integrating complex oxides on other devices, but also present enormous opportunities in recycling the deposited substrates after transfer for cost‐effective and sustainable processing of complex oxide thin films. In this work, the surface modification effects introduced on the recycled STO are investigated, and their impacts on the microstructure and properties of subsequently grown epitaxial oxide thin films are assessed and compared with those grown on the pristine substrates. Detailed analyses using high‐resolution scanning transmission electron microscopy and geometric phase analysis demonstrate distinct strain states on the surfaces of the recycled STO versus the pristine substrates, suggesting a pre‐strain state in the recycled STO substrates due to the previous deposition layer. These findings offer opportunities in growing highly mismatched oxide films on the recycled STO substrates with enhanced physical properties. Specifically, yttrium iron garnet (Y₃Fe₅O₁₂) films grown on recycled STO present different ferromagnetic responses compared to that on the pristine substrates, underscoring the effects of surface modification. The study demonstrates the feasibility of reuse and redeposition using recycled substrates. Via careful handling and preparation, high‐quality epitaxial thin films can be grown on recycled substrates with comparable or even better structural and physical properties toward sustainable process of complex oxide devices. 

Abstract Vertically aligned nanocomposite (VAN) thin films offer exceptional physical properties through diverse material combinations, providing a robust platform for designing complex nanocomposites with tailored performance. Considering materials compatibility issues, most of oxide‐metal VANs have focused on noble metals as the secondary phase in the oxide matrix. Here, an oxide‐metal hybrid metamaterials in the VAN form has been designed which combines ferroelectric BaTiO₃(BTO) with two immiscible non‐noble metal elements of Co and Cu, resulting in a three‐phase BTO‐Co‐Cu (BTO‐CC) VAN film. This film exhibits a characteristic nanopillar‐in‐matrix nanostructure with three distinct types of nanopillar morphologies, i.e., Co‐rich cylindrical nanopillars, Cu‐Co‐nanolaminated Co rectangular nanopillars and Co‐Cu‐core–shell cylindrical nanopillars. Phase field modeling indicates the constructed structure is resulted from the interplay between thermochemical, chemomechanical, and interfacial energy driving forces. The strong structural anisotropy leads to anisotropic optical and magnetic properties, presenting potential as hyperbolic metamaterial (HMM) with transverse‐positive dispersion in the near‐infrared region. The inclusion of non‐noble Cu nanostructure induces surface plasmon resonance (SPR) in the visible region. Additionally, ferroelectric properties have been demonstrated in a BTO/BTO‐CC bilayer, confirming room‐temperature multiferroicity in the film. The complex three‐phase VANs offer a novel platform for exploring electro‐magneto‐optical coupling along vertical interfaces toward future integrated devices. 

Abstract Flexible and wearable sensors show enormous potential for personalized healthcare devices by real‐time monitoring of an individual's health. Typically, a single functional material is selected for one sensor to sense a particular physical signal while multiple materials will be selected for multi‐mode sensing. Vertically aligned nanocomposites (VANs) have recently demonstrated various material combinations and novel coupled multifunctionalities that are hard to achieve in any single‐phase material alone, including multiphase multiferroics, magneto‐optic coupling, and strong magnetic and optical anisotropy. Integrating these novel VANs into wearable sensors shows enormous potential in multi‐mode sensing owing to their multifunctional nature. In this work, the transfer of VANs onto polydimethylsiloxane as a novel flexible chemical and pressure sensor is demonstrated. For this demonstration, the classical BaTiO₃‐Au VAN with combined plasmonic and piezoelectric properties is used to demonstrate a multi‐sensing mechanism. A thin water‐soluble buffer of Sr₃Al₂O₆serves as a buffer layer for the epitaxial growth and transfer process. The electrical output based on the piezoelectric responses and identifying 4‐mercaptobenzoic acid by surface‐enhanced Raman spectroscopy reveal great potential for free‐standing VANs in a wearable multifunctional sensing platform. 

Abstract The demonstration of epitaxial thin film transfer has enormous potential for thin film devices free from the traditional substrate epitaxy limitations. However, large‐area continuous film transfer remains a challenge for the commonly reported polymer‐based transfer methods due to bending and cracking during transfer, especially for highly strained epitaxial thin films. In this work, a new epoxy‐based, rigid transfer method is used to transfer films from an SrTiO₃(STO) growth substrate onto various new substrates, including those that will typically pose significant problems for epitaxy. An epitaxial multiferroic Bi₃Fe₂Mn₂O_x(BFMO) layered supercell (LSC) material is selected as the thin film for this demonstration. The results of surface and structure studies show an order of magnitude increase in the continuous area of transferred films when compared to previous transfer methods. The magnetic properties of the BFMO LSC films are shown to be enhanced by the release of strain in this method, and ferromagnetic resonance is found with an exceptionally low Gilbert damping coefficient. The large‐area transfer of this highly strained complex oxide BFMO thin film presents enormous potential for the integration of many other multifunctional oxides onto new substrates for future magnetic sensors and memory devices. 

Abstract Hybrid metamaterials (HMs) have attracted significant research interests owing to their unique optical properties and their ability to manipulate light‐matter interaction in a novel and controlled fashion beyond what any single material offers. Especially 3D HMs are of great interest due to their potential to provide advanced and precise control of such light‐matter interaction in nanoscale. In this study, a set of 3D HM nanocomposite films are designed by integrating three phases, i.e., vertically aligned CoFe₂nanosheets within the matrix of TiN/TaN multilayers. By increasing the number of TiN/TaN multilayers from 2 to 19, a high degree of tunability in optical property has been demonstrated, including well‐tailored optical permittivity, and tunable hyperbolic dispersion from Type‐II to Type‐I. Ferromagnetic CoFe₂nanosheets introduces novel magnetic responses, such as magnetic anisotropy and enhanced coercivity. Furthermore, in situ heating X‐ray diffraction (XRD) suggests good thermal stability of the 3D nanocomposite films up to the measured temperature of 600 °C. This three‐phase 3D nanocomposite design offers more flexibility in HM designs, multifunctionalities, and phase stability, compared with the typical two‐phase HMs toward future metamaterials by design. 

Abstract Hyperbolic metamaterials (HMM) possess significant anisotropic physical properties and tunability and thus find many applications in integrated photonic devices. HMMs consisting of metal and dielectric phases in either multilayer or vertically aligned nanocomposites (VAN) form are demonstrated with different hyperbolic properties. Herein, self‐assembled HfO₂‐Au/TiN‐Au multilayer thin films, combining both the multilayer and VAN designs, are demonstrated. Specifically, Au nanopillars embedded in HfO₂and TiN layers forming the alternative layers of HfO₂‐Au VAN and TiN‐Au VAN. The HfO₂and TiN layer thickness is carefully controlled by varying laser pulses during pulsed laser deposition (PLD). Interestingly, tunable anisotropic physical properties can be achieved by adjusting the bi‐layer thickness and the number of the bi‐layers. Type II optical hyperbolic dispersion can be obtained from high layer thickness structure (e.g., 20 nm), while it can be transformed into Type I optical hyperbolic dispersion by reducing the thickness to a proper value (e.g., 4 nm). This new nanoscale hybrid metamaterial structure with the three‐phase VAN design shows great potential for tailorable optical components in future integrated devices. 

Abstract Thermophotovoltaic (TPV) technology converts heat into electricity using thermal radiation. Increasing operating temperature is a highly effective approach to improving the efficiency of TPV systems. However, most reported TPV selective emitters degrade rapidly via. oxidation as operating temperatures increase. To address this issue, replacing nanostructured oxide‐metal films with oxide–oxide films is a promising way to greatly limit oxidation, even under high‐temperature conditions. This study introduces new all‐oxide photonic crystal designs for high‐temperature stable TPV systems, overcoming limitations of metal phases and offering promising material choices. The designs utilize both yttria‐stabilized zirconia (YSZ)/MgO and CeO₂/MgO combinations with a multilayer structure and stable high‐quality growth. Both designsexhibit positive optical dielectric constants with tunable reflectivity, measured via optical characterization. Thermal stability testing using in situ heating X‐ray diffraction (XRD) suggests high‐temperature stability (up to 1000 °C) of both YSZ/MgO and CeO₂/MgO systems. The results demonstrate a new and promising approach to improve the high‐temperature stability of TPV systems, which can be extended to a wide range of material selection and potential designs. 

Abstract Mott insulator VO₂exhibits an ultrafast and reversible semiconductor‐to‐metal transition (SMT) near 340 K (67 °C). In order to fulfill the multifunctional device applications, effective transition temperature (T_c) tuning as well as integrated functionality in VO₂is desired. In this study, multifunctionalities including tailorable SMT characteristics, ferromagnetic (FM) integration, and magneto‐optical (MO) coupling, have been demonstrated via metal/VO₂nanocomposite designs with controlled morphology, i.e., a two‐phase Ni/VO₂pillar‐in‐matrix geometry and a three‐phase Au/Ni/VO₂particle‐in‐matrix geometry. EvidentT_creduction of 20.4 to 54.9 K has been achieved by morphology engineering. Interestingly, the Au/Ni/VO₂film achieves a record‐lowT_cof 295.2 K (22.2 °C), slightly below room temperature (25 °C). The change in film morphology is also correlated with unique property tuning. Highly anisotropic magnetic and optical properties have been demonstrated in Ni/VO₂film, whereas Au/Ni/VO₂film exhibits isotropic properties because of the uniform distribution of Au/Ni nanoparticles. Furthermore, a strong MO coupling with enhanced magnetic coercivity and anisotropy is demonstrated for both films, indicating great potential for optically active property tuning. This demonstration opens exciting opportunities for the VO₂‐based device implementation towards smart windows, next‐generation optical‐coupled switches, and spintronic devices. 

Abstract Prussian blue analogues (PBAs) cathodes can host diverse monovalent and multivalent metal ions due to their tunable structure. However, their electrochemical performance suffers from poor cycle life associated with chemo‐mechanical instabilities. This study investigates the driving forces behind chemo‐mechanical instabilities in Ni‐ and Mn‐based PBAs cathodes for K‐ion batteries by combining electrochemical analysis, digital image correlation, and spectroscopy techniques. Capacity retention in Ni‐based PBA is 96% whereas it is 91.5% for Mn‐based PBA after 100 cycles at C/5 rate. During charge, the potassium nickel hexacyanoferrate (KNHCF) electrode experiences a positive strain generation whereas the potassium manganese hexacyanoferrate (KMHCF) electrode undergoes initially positive strain generation followed by a reduction in strains at a higher state of charge. Overall, both cathodes undergo similar reversible electrochemical strains in each charge–discharge cycle. There is ~0.80% irreversible strain generation in both cathodes after 5 cycles. XPS studies indicated richer organic layer compounds in the cathode‐electrolyte interface (CEI) layer formed on KMHCF cathodes compared to the KNHCF ones. Faster capacity fades in Mn‐based PBA, compared to Ni‐based ones, is attributed to the formation of richer organic compounds in CEI layers, rather than mechanical deformations. Understanding the driving forces behind instabilities provides a guideline to develop material‐based strategies for better electrochemical performance.