Low-Temperature Atmospheric Pressure Plasma Processes for the Deposition of Nanocomposite Coatings

Low-temperature atmospheric pressure (AP) plasma technologies have recently proven to offer a range of interesting opportunities for the preparation of a variety of nanocomposite (NC) coatings with different chemical compositions, structures, and morphologies. Since the late 2000s, numerous strategies have been implemented for the deposition of this intriguing class of coatings by using both direct and remote AP plasma sources. Interestingly, considerable progress has been made in the development of aerosol-assisted deposition processes in which the use of either precursor solutions or nanoparticle dispersions in aerosol form allows greatly widening the range of constituents that can be combined in the plasma-deposited NC films. This review summarizes the research published on this topic so far and, specifically, aims to present a concise survey of the developed plasma processes, with particular focus on their optimization as well as on the structural and functional properties of the NC coatings to which they provide access. Current challenges and opportunities are also briefly discussed to give an outlook on possible future research directions

Optical Emission Spectroscopy of Argon–Fluorocarbon–Oxygen Fed Atmospheric Pressure Dielectric Barrier Discharges

Atmospheric pressure Filamentary Dielectric Barrier Discharges (FDBDs) fed with argon, a fluorocarbon compound (i.e. CF4, C3F8, C3F6 and c-C4F8) and O2 were investigated by Optical Emission Spectroscopy (OES). The Ultra Violet-Visible (UV-Vis) spectra of the plasma in argonfluorocarbon mixtures are characterized by narrow signals from Ar atoms, CF2 and CF fragments and by two continua due to CF2+ and CF3. O2 addition to the feed gas reduces the emissions of fluorocarbon fragments and results in the appearance of an intense narrow emission at 193 nm and of a broad continuum centred at 290 nm ascribed to ArF and Ar2F excimers, respectively. Any signal from F atoms was not detected under the experimental conditions investigated even though the plasma contains F atoms since the dry etching of silicon substrates, a typical process driven by F atoms, was observed. Some possible explanations of this phenomenon are presented

Thin film deposition and surface modification with atmospheric pressure dielectric barrier discharges

This paper focuses on some of the most debated issues concerning the utilization of atmospheric pressure dielectric barrier discharges (DBDs) in surface processing of materials such as, for instance, the existence of different discharge regimes (filamentary and homogeneous), the influence on the discharge behaviour of feed gas additives and substrate properties (chemical composition, electrical characteristics, etc.). Crucial aspects of the DBD operation which highly differentiate this approach from the well established low pressure plasma technology will be discussed. An overview of the state of the art in atmospheric pressure thin film deposition from fluorocarbon- or organosilicon-containing DBDs will be also provided. In particular the possibility of tailoring the chemical composition of the coatings, the etching-deposition competition and the influence of feed gas contaminants (i.e. air and H2O) in the deposition of fluoropolymers will be discussed. Recent results on the deposition of SiOxCyHz thin films from three different methyldisiloxanes (i.e. hexamethyldisiloxane, pentamethyldisiloxane and tetramethyldisiloxane) will allow to highlight the effect of the chemical structure of the organosilicon precursor and of the oxygen-to-methyldisiloxane feed ratio on the properties of the deposits. The results obtained through different diagnostic techniques of the plasma phase (i.e. optical emission spectroscopy, GC-MS analysis of the exhaust gas) and of the deposits (i.e. XPS, FT-IR, SEM, WCA) allow to highlight interesting aspects of the fluorocarbon and organosilicon plasma chemistry at atmospheric pressure

Atmospheric Pressure PECVD of Fluorocarbon Coatings from Glow Dielectric Barrier Discharges

The influence of air and water vapour on the deposition process of fluoropolymers in argonhexafluoropropene (Ar-C3F6) filamentary dielectric barrier discharges was investigated by adding known concentrations of these contaminants to the feed gas. The obtained results show that Ar-C3F6 DBDs allow in depositing thin films with a XPS F/C ratio as high as 1.7. Under the experimental conditions investigated, contaminant addition slightly affects the F/C ratio of the coatings, and does not cause appreciable O- and N-uptake, but induces a decrease of the deposition rate. Preliminary results from the OES investigation of the gas phase and the GC-MS analysis of the gas effluent are also reported

Deposition and etching of fluorocarbon thin films in atmospheric pressure DBDs fed with Ar–CF4–H2 and Ar–CF4–O2 mixtures

The deposition and etching of plasma-polymerized fluorocarbon thin films were studied in filamentary dielectric barrier discharges (FDBDs) fed with Ar–CF4–H2 and Ar–CF4–O2 mixtures, respectively. The etching/ polymerization competition was investigated as a function of the feed composition. Hydrogen addition to CF4 promotes thin films deposition, with a maximum deposition rate at 20% H2, and reduces the F/C ratio of the deposit, while the oxygen addition promotes the etching of the plasma-deposited film. It is demonstrated that fluorine atoms can perform the etching of the fluoropolymer also without ion bombardment. The correlation between the trend of the etch rate and the trend of the surface chemical composition of fluoropolymers etched in Ar–CF4–O2 mixtures allows to enhance hypotheses on the reaction mechanism and on the role of the different active species involved in plasma–surface interactions

Fluorination of Polymers by Means of He/CF4‐Fed Atmospheric Pressure Glow Dielectric Barrier Discharges

He/CF4-fed GDBDs were used to fluorinate PP and PET. The effect of various process parameters on the polymers surface composition and morphology, as well as on their wettability was investigated inside the GDBD existence domain. An extensive surface fluorination of treated polymers due to the grafting of F atoms and CFx radicals was observed. The increase of CF4 concentration in the feed, of treatment duration, and of excitation frequency resulted in an increase of the fluorination degree corresponding to F/C ratios as high as 1.18 and 1.22 for PP and PET, respectively. The emission spectra of GDBDs fed by helium in mixture with CF4, CHF3, C2F6 and C3F8 were compared

Deposition of Hydrocarbon Films by Means of Helium‐Ethylene Fed Glow Dielectric Barrier Discharges

The present work provides a detailed study of helium-ethylene dielectric barrier glow discharges. The effect of ethylene concentration, total gas flow rate, excitation frequency and applied voltage have been investigated in order to clarify both discharge operational mode, and coating composition. The discharge has been characterized by means of electrical measurements and optical emission spectroscopy, while the stable species contained in the gas effluent have been sampled and analyzed using gas chromatography with mass spectrometry detection to achieve indications on the reactive fragments generated inside the discharge. It has been observed that polyethylene-like coatings can be obtained for a wide range of electrical conditions, and the deposition rates have been found to range between 20 and 80 nm.min-1

GC‐MS Investigation of Hexamethyldisiloxane–Oxygen Fed Cold Plasmas: Low Pressure Versus Atmospheric Pressure Operation

This study deals with the investigation by means of gaschromatography-mass spectrometry of the exhaust gas of a rf low pressure glow discharge and of an atmospheric pressure filamentary DBD fed with HMDSO, O2 and Ar. The influence of feed composition on monomer depletion and on the quali-quantitative distribution of stable by-products formed by recombination of plasma intermediates is investigated. Without O2 addition to the feed almost comparable values of HMDSO depletion are observed both at low and atmospheric pressure. Oxygen addition does not influence the monomer depletion at low pressure while it induces a slight decrease of the depletion at atmospheric pressure. Whatever the working pressure, O2 controls the overall chemistry of the plasma, since it influences the concentration of by-products (e.g. silanes, silanols, linear and cyclic methylsiloxanes). At atmospheric pressure evidences of the importance of methyl abstraction from HMDSO molecule have been obtained, while at low pressure the prevalence of Si-O bond rupture, of fragmentation and oligomerization reactions is observed. The comparison of results from the GC-MS investigation of the exhaust gas with FT-IR spectra of the deposited coatings allows to enhance hypotheses on the formation of silanols in the gas phase and in the deposit

Effect of Gas Impurities on the Operation of Dielectric Barrier Discharges Fed with He, Ar, and Ar‐C3F6

The influence of N2, O2, air, and water vapor feed gas impurities on the operation of an atmospheric pressure parallel plate DBD fed with helium and argon was investigated. The addition of increasing amounts of these impurities, under fixed excitation frequency and applied voltage, is responsible above certain thresholds of two distinct phenomena, namely the transition from a homogeneous to a filamentary appearance of the discharge and the contraction of the discharge volume. Among the different contaminants N2 shows the highest threshold limit values, O2 and H2O the lowest ones, while air generally exhibits an intermediate behavior. The effect of feed gas impurities was also studied on the PE-CVD of fluoropolymers from Ar-C3F6 fed filamentary DBDs. Contaminants addition results in a decrease of the input power and of the deposition rate as well as in a change of the film morphology, however it does not influence significantly the chemical composition of the fluoropolymer film, the monomer depletion and the distribution of by-products in the exhaust gas

Aerosol-Assisted Atmospheric Cold Plasma Deposition and Characterization of Superhydrophobic Organic–Inorganic Nanocomposite Thin Films

A facile atmospheric pressure cold plasma process is presented to deposit a novel organic–inorganic hydrocarbon polymer/ZnO nanoparticles nanocomposite coating. Specifically, this method involves the utilization of an atmospheric pressure dielectric barrier discharge (DBD) fed with helium and the aerosol of a dispersion of oleate-capped ZnO nanoparticles (NPs) in n-octane. As assessed by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance–Fourier transform infrared (ATR-FTIR) spectroscopy, the deposited nanocomposite coating combines the chemical features of both the oleate-capped ZnO NPs and the polyethylene-like organic component originated from the plasma polymerization of n-octane. Additionally, scanning electron microscopy (SEM) and transmission scanning electron microscopy (TSEM) confirm the synthesis of hierarchical micro/nanostructured coatings containing quasi-spherical NPs agglomerates. The polyethylene-like polymer covers the NPs agglomerates to different extents and contributes to their immobilization in the three-dimensional network of the coating. The increase of both the deposition time (1–10 min) and the NPs concentration in the dispersion (0.5–5 wt %) has a significant effect on the chemical and morphological structure of the thin films and, in fact, results in the increase the ZnO NPs content, which ultimately leads to superhydrophobic surfaces (advancing and receding water contact angles higher than 160°) with low hysteresis due to the hierarchical multiscale roughness of the coating