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1. Recent Advances in Vertically Aligned Nanocomposites with Tunable Optical Anisotropy: Fundamentals and Beyond

   https://doi.org/10.3390/chemosensors9060145  

   Wang, Xuejing; Wang, Haiyan (June 2021, Chemosensors)

   Developing reliable and tunable metamaterials is fundamental to next-generation optical-based nanodevices and computing schemes. In this review, an overview of recent progress made with a unique group of ceramic-based functional nanocomposites, i.e., vertically aligned nanocomposites (VANs), is presented, with the focus on the tunable anisotropic optical properties. Using a self-assembling bottom-up deposition method, the as-grown VANs present great promise in terms of structural flexibility and property tunability. Such broad tunability of functionalities is achieved through VAN designs, material selection, growth control, and strain coupling. The as-grown multi-phase VAN films also present enormous advantages, including wafer scale integration, epitaxial quality, sharp atomic interface, as well as designable materials and geometries. This review also covers the research directions with practical device potentials, such as multiplex sensing, high-temperature plasmonics, magneto-optical switching, as well as photonic circuits. 

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2. Self-assembled HfO ₂ -Au nanocomposites with ultra-fine vertically aligned Au nanopillars

   https://doi.org/10.1039/D2NR03104C  

   Zhang, Yizhi; Zhang, Di; Liu, Juncheng; Lu, Ping; Deitz, Julia; Shen, Jianan; He, Zihao; Zhang, Xinghang; Wang, Haiyan (August 2022, Nanoscale)

   Oxide-metal-based hybrid materials have gained great research interest in recent years owing to their potential for multifunctionality, property coupling, and tunability. Specifically, oxide-metal hybrid materials in a vertically aligned nanocomposite (VAN) form could produce pronounced anisotropic physical properties, e.g. , hyperbolic optical properties. Herein, self-assembled HfO 2 -Au nanocomposites with ultra-fine vertically aligned Au nanopillars (as fine as 3 nm in diameter) embedded in a HfO 2 matrix were fabricated using a one-step self-assembly process. The film crystallinity and pillar uniformity can be obviously improved by adding an ultra-thin TiN-Au buffer layer during the growth. The HfO 2 -Au hybrid VAN films show an obvious plasmonic resonance at 480 nm, which is much lower than the typical plasmonic resonance wavelength of Au nanostructures, and is attributed to the well-aligned ultra-fine Au nanopillars. Coupled with the broad hyperbolic dispersion ranging from 1050 nm to 1800 nm in wavelength, and unique dielectric HfO 2 , this nanoscale hybrid plasmonic metamaterial presents strong potential for the design of future integrated optical and electronic switching devices. 

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3. TiN–Fe Vertically Aligned Nanocomposites Integrated on Silicon as a Multifunctional Platform toward Device Applications

   https://doi.org/10.3390/cryst12060849  

   Kalaswad, Matias; Zhang, Di; Rutherford, Bethany X.; Lu, Juanjuan; Barnard, James P.; He, Zihao; Liu, Juncheng; Wang, Haohan; Xu, Xiaoshan; Wang, Haiyan (June 2022, Crystals)

   Transition metal nitrides such as titanium nitride (TiN) possess exceptional mechanical-, chemical-, and thermal-stability and have been utilized in a wide variety of applications ranging from super-hard, corrosion-resistive, and decorative coatings to nanoscale diffusion barriers in semiconductor devices. Despite the ongoing interest in these robust materials, there have been limited reports focused on engineering high-aspect ratio TiN-based nanocomposites with anisotropic magnetic and optical properties. To this end, we explored TiN–Fe thin films with self-assembled vertical structures integrated on Si substrates. We showed that the key physical properties of the individual components (e.g., ferromagnetism from Fe) are preserved, that vertical nanostructures promote anisotropic behavior, and interactions between TiN and Fe enable a special magneto-optical response. This TiN–Fe nanocomposite system presents a new group of complex multifunctional hybrid materials that can be integrated on Si for future Si-based memory, optical, and biocompatible devices. 

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4. A material design guideline for self-assembled vertically aligned nanocomposite thin films

   https://doi.org/10.1088/2515-7639/ad9bee  

   Song, Jiawei; Wang, Haiyan (January 2025, Journal of Physics: Materials)

   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. 

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5. Deposition pressure-induced microstructure control and plasmonic property tuning in hybrid ZnO–Ag _x Au _1−x thin films

   https://doi.org/10.1039/d0na00887g  

   Paldi, Robynne L.; Sun, Xing; Phuah, Xin Li; Lu, Juanjuan; Zhang, Xinghang; Siddiqui, Aleem; Wang, Haiyan (May 2021, Nanoscale Advances)

   null (Ed.)

   Self-assembled oxide–metallic alloy nanopillars as hybrid plasmonic metamaterials ( e.g. , ZnO–Ag x Au 1−x ) in a thin film form have been grown using a pulsed laser deposition method. The hybrid films were demonstrated to be highly tunable via systematic tuning of the oxygen background pressure during deposition. The pressure effects on morphology and optical properties have been investigated and found to be critical to the overall properties of the hybrid films. Specifically, low background pressure results in the vertically aligned nanocomposite (VAN) form while the high-pressure results in more lateral growth of the nanoalloys. Strong surface plasmon resonance was observed in the UV-vis region and a hyperbolic dielectric function was achieved due to the anisotropic morphology. The oxide–nanoalloy hybrid material grown in this work presents a highly effective approach for tuning the binary nanoalloy morphology and properties through systematic parametric changes, important for their potential applications in integrated photonics and plasmonics such as sensors, energy harvesting devices, and beyond. 

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