Chemical Vapor Deposition Polymerization Research Articles

Air-Stable Wafer-Scale Ferromagnetic Metallo-Carbon Nitride Monolayer.

The pursuit of robust, long-range magnetic ordering in two-dimensional (2D) materials holds immense promise for driving technological advances. However, achieving this goal remains a grand challenge due to enhanced quantum and thermal fluctuations as well as chemical instability in the 2D limit. While magnetic ordering has been realized in atomically thin flakes of transition metal chalcogenides and metal halides, these materials often suffer from air instability. In contrast, 2D carbon-based materials are stable enough, yet the challenge lies in creating a high density of local magnetic moments and controlling their long-range magnetic ordering. Here, we report a novel wafer-scale synthesis of an air-stable metallo-carbon nitride monolayer (MCN, denoted as MN4/CNx), featuring ultradense single magnetic atoms and exhibiting robust room-temperature ferromagnetism. Under low-pressure chemical vapor deposition conditions, thermal dehydrogenation and polymerization of metal phthalocyanine (MPc) on copper foil at elevated temperature generate a substantial number of nitrogen coordination sites for anchoring magnetic single atoms in monolayer MN4/CNx (where M = Fe, Co, and Ni). The incorporation of densely populating MN4 sites into monolayer MCN networks leads to robust ferromagnetism up to room temperature, enabling the observation of anomalous Hall effects with excellent chemical stability. Detailed electronic structure calculations indicate that the presence of high-density metal sites results in the emergence of spin-split d-bands near the Fermi level, causing a favorable long-range ferromagnetic exchange coupling through direct exchange interactions. Our work demonstrates a novel synthesis approach for wafer-scale MCN monolayers with robust room-temperature ferromagnetism and may shed light on practical electronic and spintronic applications.

Emergent Properties, Functions, and Applications of Phane‐Based Polymers

AbstractOver the past few decades, chemical vapor deposition (CVD) of [2.2]paracyclophanes has captured significant attention as an emergent technology, producing conformal, chemically pure, and pinhole‐free coatings for biomedical and industrial applications. Compelling examples range from functional CVD polymers to tailored nanostructures. In this work, the unique functional properties of polymers derived from [2.2]paracyclophanes are connected with emergent applications. Special attention is given to the function‐property relationships in the areas of electronic materials, biomaterials, and separation materials. A particular focus is to highlight the versatility of CVD polymerization to process these polymers.

Molecular Insights into [2.2]Paracyclophane‐Based Functional Materials: Chemical Aspects Behind Functions

AbstractMany of the functions and features of practically useful materials are the province of molecular‐level chemistry and their modulation at different length‐scale. This report illustrates the molecular‐level chemistry behind functions and features of the [2.2]paracyclophane‐based materials with a particular focus on the most recent explorations on through‐space conjugated small‐molecule organic emitters, π‐stacked macrocyclic molecules and polymers, poly(p‐phenylenevinylene)s featuring well‐defined donor‐acceptors sequence control, and surface engineering of technologically‐relevant parylenes that finds broad applications across the field of chemical science and technology. This report largely deals with the potential and opportunities associated with molecular features and functions of planar chirality, conformational behaviors, strain‐induced non‐planarity of the aromatics, the profound impacts of through‐space conjugation and π‐electron interactions/delocalization on optoelectronic properties of the π‐conjugated organic emitters, polymers and extended structures consisting of cyclophanes. A special focus is put on the concept of supramolecular polymers using chemically‐programmed chiral cyclophanes via non‐covalent stacking and controlled conformational arrangements. Illustrating cyclophane as precursors/monomers and fabrication strategies for their incorporation in structurally‐controlled (poly(p‐xylylene)s formed via chemical vapor deposition polymerization and post‐deposition fabrication for interface engineering is described. Demonstrating a rather different approach of electronically‐dictated ring‐opening metathesis polymerization employing strained cyclophane‐diene precursors that generate conjugated poly(p‐phenylenevinylene)s with well‐defined (i.e., low dispersity) and donor‐acceptor sequence control is also discussed. This report will serve as an indispensable one‐stop reference for organic, and polymer chemists, as well as material scientists working with cyclophanes for research innovations.

Easily scaled-up and portable 3D polysulfone hollow fiber membrane tree for high-efficient solar-driven clean water production

Solar-driven water evaporation based on photothermal conversion materials, such as woods, sponges, and membranes, have been proved to be promising in the field of fresh water generation. It's urgent to develop portable, versatile, and efficient photothermal materials for affordable freshwater supply. Here, a hydrophilic polysulfone hollow fiber membrane (PSF HFM) with gradient porous structure and diameter of 50 μm was prepared through one-step phase inversion method. Inspired by nature, a highly efficient 3D solar-driven steam evaporator was fabricated via chemical vapor deposition polymerization (CVDP) of hydrophobic polypyrrole (PPy) on PSF HFMs. With rational 3D structure, capillary-driven water transportation property, optical absorption as high as 95%, heat localization, photothermal conversion, a solar-driven water evaporation rate of 2.6 kg m−2·h−1, and the conversion efficiency of 173.2% could be achieved. Furthermore, the HFM tree with affluent light-harvesting, efficient thermal management, and excellent stability showed a purified water collection efficiency of 14.18 L/(m2·day) in practical application, providing a latent way to boost water purification, regeneration, and desalination.

Chemical and Topological Control of Surfaces Using Functional Parylene Coatings

Chemical vapor deposition (CVD) polymerization is a prevalent technique for fabricating conformal, defect-free, and systematically adjustable organic thin films. CVD is particularly beneficial for barrier coatings due to its ability to eliminate solvent-related environmental, health, and safety risk factors and provide a wide spectrum of post-polymerization modification strategies. This review discusses poly-p-xylylene and its functional derivatives. CVD polymerization of [2.2]paracyclophane precursors has undergone a recent renaissance due to advancements in chemical and morphological surface manipulation. This review summarizes emerging trends based on the following outline:Table of content:1 Introduction2 CVD Polymerization as a Sustainable Coating Technology3 CVD Instrumentation4 Poly-p-xylylene Coatings: Background of Polymerization Process and Functionalized Films5 Main Applications of Poly-p-xylylenes6 Area-Selective CVD Polymerization7 Fabrication and Applications of Topological Structures8 Conclusions and Outlook

Systematic Studies into the Area Selectivity of Chemical Vapor Deposition Polymerization

As the current top-down microchip manufacturing processes approach their resolution limits, there is a need for alternative patterning technologies that offer high feature densities and edge fidelity with single-digit nanometer resolution. To address this challenge, bottom-up processes have been considered, but they typically require sophisticated masking and alignment schemes and/or face materials' compatibility issues. In this work, we report a systematic study into the impact of thermodynamic processes on the area selectivity of chemical vapor deposition (CVD) polymerization of functional [2.2]paracyclophanes (PCP). Adhesion mapping of preclosure CVD films by atomic force microscopy (AFM) provided a detailed understanding of the geometric features of the polymer islands that form under different deposition conditions. Our results suggest a correlation between interfacial transport processes, including adsorption, diffusion, and desorption, and thermodynamic control parameters, such as substrate temperature and working pressure. This work culminates in a kinetic model that predictes both area-selective and nonselective CVD parameters for the same polymer/substrate ensemble (PPX-C + Cu). While limited to a focused subset of CVD polymers and substrates, this work provides an improved mechanistic understanding of area-selective CVD polymerization and highlights the potential for thermodynamic control in tuning area selectivity.

Emergence of Structural Phosphorescence in Free-Standing, Laterally Organized Polymer Nanofiber Membranes

In the past decade, the phosphorescence of metal-free confined polymer structures has generated interest in optoelectronics due to their sustainability, low toxicity, and deployment in sensing applications. Herein, a free-standing array of laterally organized nanofibers was prepared via templated chemical vapor deposition polymerization into a liquid crystalline (LC) phase, and their optical properties were compared to compositionally identical films. The fibers converge into laterally aligned membranes that maintain a high internal order despite the surfaces of the membranes remaining uniform and closed. The membranes consisted of hourglass-shaped nanofibers with features in the micro- and nanometer regime. Free-standing nanofiber membranes differ from polymer films of equivalent chemical composition in several key features: (1) Anisotropic growth of polymer nanofibers with constraint and compliance to an LC template, (2) a high surface-to-volume ratio, and (3) the occurrence of a long-lived emission in the blue region, which persists for multiple seconds after excitation. This study constitutes the first report of long-lived emission from solid-state poly(p-xylylenes) nanofibers. Prospective applications of morphologically controlled polymer arrays with structural luminescence include organic sensors and optoelectronic devices.

Significantly Improved Dielectric Performance of All-Organic Parylene/Polyimide/Parylene Composite Films with Sandwich Structure.

The development of novel polymer dielectrics with enhanced dielectric performance is a great challenge for application of film capacitors in modern electronics and electrical systems. Herein, an innovative approach of chemical vapor deposition polymerization technology is proposed to prepare the all-organic sandwich structured parylene/polyimide/parylene (Py/PI/Py) composite films by employing poly(chloro-para-xylylene) (parylene C) as the outer layers and polyimide (PI) as the inner layer. The Py/PI/Py composites exhibit superior thermal resistance and outstanding mechanical properties. Moreover, thanks to the interfacial effect which contributes to reinforcing the dielectric response and the thickness effect which facilitates improving the breakdown strength, the dielectric performance of Py/PI/Py composites has been enhanced significantly. Accordingly, dielectric constant of 4.52-5.09, dissipation factor of 0.21-1.01%, and breakdown strength of 307-460MVm-1 are achieved. Besides, notable energy storage performance is also obtained in Py/PI/Py composite dielectrics. Consequently, this novel application of chemical vapor deposition polymerization method in preparing all-organic multilayered polymer composite films with sandwich structure shows promising potential in film capacitor applications in harsh conditions.

Design Strategies for Structurally Controlled Polymer Surfaces via Cyclophane-Based CVD Polymerization and Post-CVD Fabrication.

Molecular structuring of soft matter with precise arrangements over multiple hierarchical levels, especially on polymer surfaces, and enabling their post-synthetic modulation has tremendous potential for application in molecular engineering and interfacial science. Here, recent research and developments in design strategies for structurally controlled polymer surfaces via cyclophane-based chemical vapor deposition (CVD) polymerization with precise control over chemical functionalities and post-CVD fabrication via orthogonal surface functionalization that facilitates the formation of designable biointerfaces are summarized. Particular discussion about innovative approaches for the templated synthesis of shape-controlled CVD polymers, ranging from 1D to 3D architecture, including inside confined nanochannels, nanofibers/nanowires synthesis into an anisotropic media such as liquid crystals, and CVD polymer nanohelices via hierarchical chirality transfer across multiple length scales is provided. Aiming at multifunctional polymer surfaces via CVD copolymerization of multiple precursors, the structural and functional design of the fundamental [2.2]paracyclophane (PCP) precursor molecules, that is, functional CVD monomer chemistry is also described. Technologically advanced and innovative surface deposition techniques toward topological micro- and nanostructuring, including microcontact printing, photopatterning, photomask, and lithographic techniques such as dip-pen nanolithography, showcasing research from the authors' laboratories as well as other's relevant important findings in this evolving field are highlighted that have introduced new programmable CVD polymerization capabilities. Perspectives, current limitations, and future considerations are provided.

BMP Gene-Immobilization to Dental Implants Enhances Bone Regeneration.

For individuals who have experienced tooth loss, dental implants are an important treatment option for oral reconstruction. For these patients, alveolar bone augmentation and acceleration of osseointegration optimize implant stability. Traditional oral surgery often requires invasive procedures, which can result in prolonged treatment time and associated morbidity. It has been previously shown that chemical vapor deposition (CVD) polymerization of functionalized [2.2]paracyclophanes can be used to anchor gene encoding vectors onto biomaterial surfaces and local delivery of a bone morphogenetic protein (BMP)-encoding vector can increase alveolar bone volume and density in vivo. This study is the first to combine the use of CVD technology and BMP gene delivery on titanium for the promotion of bone regeneration and bone to implant contact in vivo. BMP-7 tethered to titanium surface enhances osteoblast cell differentiation and alkaline phosphatase activity in vitro and increases alveolar bone regeneration and % bone to implant contact similar to using high doses of exogenously applied BMP-7 in vivo. The use of this innovative gene delivery strategy on implant surfaces offers an alternative treatment option for targeted alveolar bone reconstruction.

Surfaces Decorated with Enantiomorphically Pure Polymer Nanohelices via Hierarchical Chirality Transfer across Multiple Length Scales.

Mesoscale chiral materials are prepared by lithographic methods, assembly of chiral building blocks, and through syntheses in the presence of polarized light. Typically, these processes result in micrometer-sized structures, require complex top-down manipulation, or rely on tedious asymmetric separation. Chemical vapor deposition (CVD) polymerization of chiral precursors into supported films of liquid crystals (LCs) are discovered to result in superhierarchical arrangements of enantiomorphically pure nanofibers. Depending on the molecular chirality of the 1-hydroxyethyl [2.2]paracyclophane precursor, extended arrays of enantiomorphic nanohelices are formed from achiral nematic templates. Arrays of chiral nanohelices extend over hundreds of micrometers and consistently display enantiomorphic micropatterns. The pitch of individual nanohelices depends on the enantiomeric excess and the purity of the chiral precursor, consistent with the theoretical model of a doubly twisted LC director configuration. During CVD of chiral precursors into cholesteric LC films, aspects of molecular and mesoscale asymmetry combine constructively to form regularly twisted nanohelices. Enantiomorphic surfaces permit the tailoring of a wide range of functional properties, such as the asymmetric induction of weak chiral systems.

Ultra-robust vertically aligned three-dimensional (3D) Janus hollow fiber membranes for interfacial solar-driven steam generation with salt-resistant and multi-media purification

The emerging solar-driven steam generation technology is considered as one of the most promising strategies for purifying seawater and wastewater, while it is still challenging to achieve superior evaporation rate, salt-resistant property and multi-media evaporation applications at the same time. Herein, an ultra-robust Janus PP hollow fiber membrane (PP HFM) with capillary-driven siphon and photothermal properties was fabricated. Dopamine (DA), (3-aminopropyl) triethoxy-silane (KH550), TiO2 nanoparticles were immobilized on the inner surface of PP HFMs for turning from a common hydrophobic state to a hydrophilic state. An outer surface with hydrophobicity is constructed by chemical vapor deposition polymerization (CVDP) of polypyrrole (PPy). The fabricated PP-PDA/KH550/TiO2-PPy HFMs showed an asymmetric wettability (inner and outer water contact angle of 0° and 95°), mechanical strength of 1.2 GPa, and light absorption of 95%. Importantly, the low heat loss through decreasing contact area with water enabled vertically aligned HFMs with a superior solar evaporation rate of 3.65 kg/(m2·h), and a high photothermal conversion efficiency of 229.13% under one-sun irradiation. Furthermore, the Janus PP HFMs showed a superior salt-resistance without salt crystals even in 20 wt% high salinity condition, rejections of ion (<10 ppm salt ions), oil (>99%), and dye (>99%), and excellent stability. The Janus PP HFMs evaporator provide a new paradigmfor low heat loss, high evaporation rate, salt-resistant and multi-media purification.

Vapor-Phase Fabrication of a Maleimide-Functionalized Poly-p-xylylene with a Three-Dimensional Structure

A vapor-phase process, involving the sublimation of an ice substrate/template and the vapor deposition of a maleimide-functionalized poly-p-xylylene, has been reported to synthesize an advanced porous material, with readily clickable chemical interface properties, to perform a Michael-type addition of a maleimide functionality for conjugation with a thiol group. In contrast to the conventional chemical vapor deposition of poly-p-xylylenes on a solid surface that forms thin film coatings, the process reported herein additionally results in deposition on a dynamic and sublimating ice surface (template), rendering the construction of a three-dimensional, porous, maleimide-functionalized poly-p-xylylene. The process seamlessly exploits the refined chemical vapor deposition polymerization from maleimide-substituted [2,2]paracyclophane and ensures the preservation and transformation of the maleimide functionality to the final porous poly-p-xylylene products. The functionalization and production of a porous maleimide-functionalized poly-p-xylylene were completed in a single step, thus avoiding complicated steps or post-functionalization procedures that are commonly seen in conventional approaches to produce functional materials. More importantly, the equipped maleimide functionality provides a rapid and efficient route for click conjugation toward thiol-terminated molecules, and the reaction can be performed under mild conditions at room temperature in a water solution without the need for a catalyst, an initiator, or other energy sources. The introduced vapor-based process enables a straightforward synthesis approach to produce not only a pore-forming structure of a three-dimensional material, but also an in situ-derived maleimide functional group, to conduct a covalent click reaction with thiol-terminal molecules, which are abundant in biological environments. These advanced materials are expected to have a wide variety of new applications.

Vapor Sublimation and Deposition to Fabricate a Porous Methyl Propiolate-Functionalized Poly-p-xylylene Material for Copper-Free Click Chemistry.

Conventional porous materials are mostly synthesized in solution-based methods involving solvents and initiators, and the functionalization of these porous materials usually requires additional and complex steps. In the current study, a methyl propiolate-functionalized porous poly-p-xylylene material was fabricated based on a unique vapor sublimation and deposition process. The process used a water solution and ice as the template with a customizable shape and dimensions, and the conventional chemical vapor deposition (CVD) polymerization of poly-p-xylylene on such an ice template formed a three-dimensional, porous poly-p-xylylene material with interconnected porous structures. More importantly, the functionality of methyl propiolate was well preserved by using methyl propiolate-substituted [2,2]-paracyclophane during the vapor deposition polymerization process and was installed in one step on the final porous poly-p-xylylene products. This functionality exhibited an intact structure and reactivity during the proposed vapor sublimation and deposition process and was proven to have no decomposition or side products after further characterization and conjugation experiments. The electron-withdrawing methyl propiolate group readily provided efficient alkynes as click azide-terminated molecules under copper-free and mild conditions at room temperature and in environmentally friendly solvents, such as water. The resulting methyl propiolate-functionalized porous poly-p-xylylene exhibited interface properties with clickable specific covalent attachment toward azide-terminated target molecules, which are widely available for drugs and biomolecules. The fabricated functional porous materials represent an advanced material featuring porous structures, a straightforward synthetic approach, and precise and controlled interface click chemistry, rendering long-term stability and efficacy to conjugate target functionalities that are expected to attract a variety of new applications.

Tailoring Electrode-Electrolyte Interfaces in Lithium-Ion Batteries Using Molecularly Engineered Functional Polymers.

Electrode-electrolyte interfaces (EEIs) affect the rate capability, cycling stability, and thermal safety of lithium-ion batteries (LIBs). Designing stable EEIs with fast Li+ transport is crucial for developing advanced LIBs. Here, we study Li+ kinetics at EEIs tailored by three nanoscale polymer thin films via chemical vapor deposition (CVD) polymerization. Small binding energy with Li+ and the presence of sufficient binding sites for Li+ allow poly(3,4-ethylenedioxythiophene) (PEDOT) based artificial coatings to enable fast charging of LiCoO2. Operando synchrotron X-ray diffraction experiments suggest that the superior Li+ transport property in PEDOT further improves current homogeneity in the LiCoO2 electrode during cycling. PEDOT also forms chemical bonds with LiCoO2, which reduces Co dissolution and inhibits electrolyte decomposition. As a result, the LiCoO2 4.5 V cycle life tested at C/2 increases over 1700% after PEDOT coating. In comparison, the other two polymer coatings show undesirable effects on LiCoO2 performance. These insights provide us with rules for selecting/designing polymers to engineer EEIs in advanced LIBs.

Dopamine-assisted chemical vapour deposition of polypyrrole on graphene for flexible supercapacitor

Developing high-performance carbon-based flexible supercapacitors is crucial for energy-storage systems with high stability and safety. Here we report a succinct method of fabricating polypyrrole (PPY) by chemical vapour deposition polymerization on flexible graphene substrate. The dependency of initiators such as FeCl3 and CuCl2 for the polymerization of pyrrole is elucidated as well. The morphological changes as well as doping concentrations of PPY affect the capacitive behaviours of the nanocomposites. In addition, an assistance of dopamine between PPY and graphene not only improves the uniformity of PPY layers, but also enhances the supercapacitor performance. These results demonstrate the potential for the wearable energy-storage devices application.

Molecular Changes in Vapor‐Based Polymer Thin Films Assessed by Characterization of Swelling Properties of Amine‐Functionalized Poly‐p‐xylylene

AbstractThough widely applied in various technological areas, the knowledge about the molecular behavior of poly‐p‐xylylene and its derivatives prepared via chemical vapor deposition polymerization is still far from comprehensive, as for this polymer the characterization is limited due to the insolubility in most common solvents. This report presents the investigation of the swelling properties of aminomethyl‐functionalized poly‐p‐xylylene as a means to elucidate the influence of various process parameters on the structural properties of the polymer layer. The swelling of films with varying thickness, storage time and conditions, thermal treatment and deposited by varying the temperature of the deposition stage is measured. To gain deeper insight into the origin of the observed effects, the swelling measurements by spectroscopic ellipsometry are complemented using additional characterization methods.

Synthesis of MWCNT/PPY nanocomposite using oxidation polymerization method and its employment in sensing such as CO2 and humidity

This paper includes the synthesis of a composite of multiwall carbon nanotube (MWCNT) and conducting polymer polypyrrol (PPY) and its characterizations along with the applications direct liquid injection chemical vapour deposition (DLICVD) and oxidation polymerization method have been used for the synthesis of MWCNT and MWCNT/PPY nanocomposite respectively. MWCNT/PPY's thin film was prepared using the technique of spin coating and characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), particle size analyzer and X-ray diffractometer (XRD). Raman spectroscopy has been used to observe the vibrational and rotational spectra of MWCNT and MWCNT/PPY nanocomposite. The optical bandgap and minimum crystallite size have been calculated and found to be 3.2 eV and 8.1 nm respectively. The synthesized MWCNT/PPY has been used for CO2 and humidity sensing at room temperature (300 K). The sensor response of thin-film towards CO2 was found to be 7.2 at 1000 ppm and minimum response and recovery time at 250 ppm were found to be 30 s and 37 s respectively. The sensitivity of thin-film towards humidity was found to be 41.33 kΩ/%RH. The theoretical calculation was also performed in support of experimental data.

Nanoscale control by chemically vapour-deposited polymers

Chemical vapour deposition (CVD) enables nanoscale control for the synthesis of high-purity polymer thin films. Ultrathin (<20 nm) and ultrasmooth (<1 nm r.m.s. roughness) layers of CVD polymers can conform to the geometry of the growth surface. This Review focuses on CVD polymerization methods adapted from solution chemistry for selectively forming different classes of macromolecules. The mechanistically based CVD approaches provide full retention of the monomer’s organic functional groups and thus provide a rational basis for designing and optimizing film characteristics for a diverse array of applications. These include conjugated polymers for energy storage, thin dielectrics for low-power, flexible devices and responsive hydrogels for controlled drug release. Systematic variation in the CVD process parameters provides remarkable control of π–π stacking in conducting polymers, mesh sizes in hydrogels, crystallographic texture, surface energy, permeation rates of molecules and ions, optoelectronic properties and switchable smart behaviour. The initial research focus on process fundamentals, including adsorption, reaction kinetics, mass transport and conformality, formed a strong basis for the recent rapid expansion of materials, applications and scale-up activities in multiple laboratories. The materials and approaches used in CVD polymerization are also extending into hybrid inorganic/organic materials and devices. Chemical vapour deposition (CVD) enables the synthesis of high-purity, pinhole-free and conformal polymer thin films. This Review discusses the recent breakthroughs in mechanistically based CVD polymerization processes and designing CVD polymers for a diverse array of applications.

Poly-para-xylylene enhanced Fe-based amorphous powder cores with improved soft magnetic properties via chemical vapor deposition

In this work, dense and pinhole-free poly-para-xylylene (PPX) with high thermal stability, high insulating property and high lubricity has been designed as coating layer for Fe-based amorphous powder. Three ultra-thin PPX layers have been successfully deposited on the surface of FeSiBPNbCr amorphous powder via chemical vapor deposition (CVD) polymerization. The micromorphology and chemical structure of the PPX layer on the powder surface, as well as the conversion mechanisms, has been investigated systematically. Several methods have been used to estimate the amount and thickness of PPX layers. The corresponding amorphous powder cores (AMPCs) were also prepared. With the increasing thickness of the PPX layer, the saturation magnetization (Ms) of the powder decreased slightly but the magnetic flux density (Bs) of the AMPCs kept at a similar level thanked to the enhanced green density. In addition, the DC bias performance was improved and core loss (Pcv) was reduced. Compare with other coating materials like epoxy and silicon resin, the PPX layer provided better insulation and higher lubricity for the AMPCs, thus reduced the Pcv and improved the DC bias performance. The AMPCs coated by PPX-3 exhibited excellent comprehensive soft magnetic properties, which were promising for high-power and high-frequency applications.