Heung-Sik Tae

The role of precursor chemistry in determining the charge-transport characteristics of APPJ-derived polypyrrole (PPy) nanostructured films was assessed using two precursor formulations. One consisted of untreated liquid pyrrole monomer (Type I), whereas the other was a liquid plasma-treated (PT) pyrrole polymer (Type II) that had undergone plasma pretreatment. Particular attention was given to the relationship between precursor characteristics, film morphology, and the resulting electrical performance, especially the formation of a compact and highly interconnected polymer network. The OES and ICCD observations indicate that the Type II precursor undergoes more effective plasma activation, which is associated with enhanced electron production during ionization processes while retaining plasma-induced oligomer characteristics. The resulting increase in plasma excitation promotes the generation of reactive excited species, facilitating the chemical reactions required for subsequent PPy polymerization. In addition, the morphological and chemical analyses using field emission-scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) revealed distinct differences in the Type II-derived PPy layer. The film developed a compact and highly interconnected nanoscale architecture, accompanied by characteristic C–N and C=C bonding features identified from the spectroscopic analysis. These structural and chemical characteristics are likely responsible for maintaining continuous charge-transport pathways within the polymer network. Consequently, the Type II-based film retained its electrical conductivity more effectively during prolonged exposure to ambient conditions than the corresponding Type I film.

This review presents recent advancements in cold atmospheric pressure (AP) plasma (CAP) processes for the synthesis and surface treatment of polymer films and metal nanoparticles (NPs) in biomedical applications. We discuss the properties and applications of atmospheric pressure plasma (APP) processes, including dielectric barrier discharge (DBD) and plasma jet methods, highlighting their effectiveness in controlling surface characteristics such as wettability and functionalization.

In this review, we introduce recently developed plasma-based approaches for depositing and treating piezoelectric nanoparticles (NPs) and piezoelectric polymer films for nanogenerator (NG) and sensor applications. We also present the properties and an overview of recently synthesized or modified piezoelectric materials on piezoelectric polymers to highlight the existing challenges and future directions of plasma methods under vacuum, low pressure, and ambient air conditions. The various plasma processes involved in piezoelectric NGs and sensors, including plasma-based vapor deposition, dielectric barrier discharge, and surface modification, are introduced and summarized for controlling various surface properties (etching, roughening, crosslinking, functionalization, and crystallinity).

Beyond the existing in-solution plasma (iSP) process used for nanoparticle synthesis, polymer film synthesis has recently been demonstrated via iSP, which occurs at a specific cycle in a substrate with electrical potential. Herein, we propose an iSP reactor designed to tune the characteristics of polypyrrole (PPy) films by applying varied DC biases to the substrate during the solution plasma process. Applying DC bias to the substrate notably enhances the iSP during the negative cycle, thereby generating ionic precursors from pyrrole monomers for PPy film formation. These films demonstrated morphological variances as a function of the applied VDC. At a low voltage (VDC = 0.5 kV), a uniform film was formed via a layer-by-layer growth mechanism, and as the voltage was increased, the film exhibited a structured morphology. Specifically, at VDC = 0.5 kV, the PPy film was successfully coated onto a finely patterned electrode. The proposed iSP process, under specific DC biasing conditions, enables polymer coating without necessitating additional chemicals, catalysts, or masks. This method holds potential for a wide range of applications.

To improve the electrical conductivity of polypyrrole (PPy) nanostructure film through in situ iodine (I₂) doping, this study proposes an atmospheric pressure plasma reactor (APPR) where heated I₂ dopant vapor is fed through capillary electrodes that serve as electrodes for discharge ignition. A large amount of the heated I₂ vapor introduced into the reactor separately from a monomer gas can be effectively activated by an intense plasma via capillary electrodes. In particular, intensive plasma is obtained by properly adjusting the bluff body position in the APPR. Based on the ICCD and OES results, the I₂ vapor injected through the capillary nozzle electrode is observed to form I₂ charge species. The formed I₂ species could directly participate in growing in situ I₂-doped PPy films. Thus, in situ I₂-doped PPy nanostructure films grown using the proposed APPR exhibit higher thicknesses of 15.3 μm and good electrical conductivities, compared to the corresponding non-doped films.

To generate a stable and effective air–liquid discharge in an open atmosphere, we investigated the effect of the dielectric barrier on the discharge between the pin electrode and liquid surface in an atmospheric-pressure plasma reactor. The atmospheric-pressure plasma reactor used in this study was based on a pin–plate discharge structure, and a metal wire was used as a pin-type power electrode. A plate-type ground electrode was placed above and below the vessel to compare the pin–liquid discharge and pin–liquid barrier discharge (PLBD). The results indicated that the PLBD configuration utilizing the bottom of the vessel as a dielectric barrier outperformed the pin–liquid setup in terms of the discharge stability and that the concentration of reactive species was different in the two plasma modes. PLBD can be used as a digestion technique for determining the phosphorus concentration in natural water sources. The method for decomposing phosphorus compounds by employing PLBD exhibited excellent decomposition performance, similar to the performance of thermochemical digestion—an established conventional method for phosphorus detection in water. The PLBD structure can replace the conventional chemical-agent-based digestion method for determining the total dissolved phosphorus concentration using the ascorbic acid reduction method.

Herein, we describe a new flexible atmospheric pressure plasma jet device composed of hollow-core optical fibers and introduce two potential applications of the device: endoscopic plasma treatment and decomposition of aqueous phosphorus compounds. The proposed device is 1.6 m long and highly flexible, has a small probe size of several hundred micrometers, and spatially separates the plasma jet from the electrical input, making it very suitable for treatment through the biopsy channel of a conventional endoscope. Because the wire electrode of the fabricated atmospheric pressure plasma jet device is thoroughly isolated inside the hollow of the optical fiber, the device produces a plasma column in an identical discharge state regardless of external environmental conditions. As a result, the device can operate safely and steadily in highly humid environments, such as underwater, which can be utilized to decompose phosphorus compounds in fresh water.

This study investigates the structural phase and dielectric properties of poly(vinylidenefluoride-co-trifluoroethylene) (P[VDF–TrFE]) thin films grown via atmospheric pressure (AP) plasma deposition using a mixed polymer solution comprising P[VDF–TrFE] polymer nano powder and dimethylformamide (DMF) liquid solvent. The length of the glass guide tube of the AP plasma deposition system is an important parameter in producing intense cloud-like plasma from the vaporization of DMF liquid solvent containing polymer nano powder. This intense cloud-like plasma for polymer deposition is observed in a glass guide tube of length 80 mm greater than the conventional case, thus uniformly depositing the P[VDF–TrFE] thin film with a thickness of 3 μm. The P[VDF–TrFE] thin films with excellent β-phase structural properties were coated under the optimum conditions at room temperature for 1 h. However, the P[VDF–TrFE] thin film had a very high DMF solvent component. The post-heating treatment was then performed on a hotplate in air for 3 h at post-heating temperatures of 140 °C, 160 °C, and 180 °C to remove DMF solvent and obtain pure piezoelectric P[VDF–TrFE] thin films. The optimal conditions for removing the DMF solvent while maintaining the β phases were also examined. The post-heated P[VDF–TrFE] thin films at 160 °C had a smooth surface with nanoparticles and crystalline peaks of β phases, as confirmed by the Fourier transform infrared spectroscopy and XRD analysis. The dielectric constant of the post-heated P[VDF–TrFE] thin film was measured to be 30 using an impedance analyzer at 10 kHz and is expected to be applied to electronic devices such as low-frequency piezoelectric nanogenerators.

Polyaniline (PANI) was synthesized from liquid aniline, a nitrogen-containing aromatic compound, through the atmospheric pressure (AP) plasma process using a newly designed plasma jet array with wide spacing between plasma jets. To expand the area of the polymerized film, the newly proposed plasma jet array comprises three AP plasma jet devices spaced 7 mm apart in a triangular configuration and an electrodeless quartz tube capable of applying auxiliary gas in the center of the triangular plasma jets. The vaporized aniline monomer was synthesized into a PANI film using the proposed plasma array device. The effects of nitrogen gas addition on the morphological, chemical, and electrical properties of PANI films in AP argon plasma polymerization were examined. The iodine-doped PANI film was isolated from the atmosphere through encapsulation. The constant electrical resistance of the PANI film indicates that the conductive PANI film can achieve the desired resistance by controlling the atmospheric exposure time through encapsulation.