ATMOSPHERIC PRESSURE TOWNSEND DISCHARGES AS APROMISING TOOL FOR THE ONE-STEP DEPOSITION OF ANTIFOGGINGCOATINGS FROM N2O/TMCTS MIXTURES

Iván Rodríguez Durán 1,2, Antoine Durocher-Jean 3, Jacopo Profili 3, Luc Stafford 3, Gaétan Laroche 1,2

1Laboratoire d’ingénierie de surface, Centre de Recherche sur les Matériaux Avancés, Département de génie des mines, de la métallurgie et des matériaux, 1065 avenue de la médecine, Université Laval, Québec, G1V 0A6, Canada.

2Centre de recherche du CHU de Québec, Axe Médecine Régénératrice, Hôpital Saint-François d’Assise, 10 rue de l’Espinay, Québec, G1L 3L5, Canada.

3Département de Physique, Université de Montréal, Complexe des sciences - B-2043, 1375 avenue Thérèse-Lavoie-Roux, Montréal H2V 0B3, Canada

ABSTRACT The need to ensuring the “see-through” property of transparent materials when exposed to sudden temperature changes or very humid conditions has encouraged the development of antifogging strategies, such as the deposition of (super) hydrophilic coatings. However, despite the effectiveness of these coatings in combating the effects of fogging, most of the coating techniques explored to date are typically time-consuming and environment-unfriendly. Bearing this in mind, we demonstrate that the application of dielectric barrier discharges operated at atmospheric pressure proves to be successful in preparing antifogging coatings on glass samples from 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) and nitrous oxide (N2O). The antifogging performance of the coatings was found to be governed by the [N2O]/[TMCTS] ratio and not by the [N2O] + [TMCTS] sum. Coatings prepared under a [N2O]/[TMCTS] = 30 were superhydrophilic (water contact angles ≈ 5°–10°) due to surface silanol groups and endowed glass samples with a superior antifogging property, as revealed by the ASTM F 659-06 test. In contrast, because of the lesser hydrophilicity (water contact angles ≈ 60°), coatings prepared under a [N2O]/[TMCTS] = 10 did not endow glass samples with antifogging property. Regardless of the deposition conditions, the plasma-deposited coatings displayed crack-free smooth surfaces (Rrms = 2−4 nm).

KEYWORDS antifogging coatings, dielectric barrier discharges (DBD), surface composition, tetramethylcyclotetrasiloxane (TMCTS), wettability

CITATION I. Rodriguez Duran, A. Durocher-Jean, J. Profili, L. Stafford, and G. Laroche. Atmospheric pressure Townsend discharges as a promising tool for the one-step deposition of antifogging coatings from N2O/TMCTS mixtures, Plasma Processes, 2020;17:e1900186 1-15.

This is the author’s version of the original manuscript. The final publication is available via DOI: https://doi.org/10.1002/ppap.201900186

1 INTRODUCTION

Fogging is a natural phenomenon in which water vapor condenses into the form of tiny droplets on a solid substrate, whose temperature falls below the dew point.[1] The resulting myriad of water drops scatter the incident light in all directions, leading to the formation of a typical “whitish layer” one can observe, for instance, on bathroom mirrors after having a shower.[2] The effects of this surface condensation have been shown to depend on both the morphology and the size of water drops. As a rule, the higher the contact angle and the smaller the drops, the more pronounced the effects of fogging are.[3]

The effects of condensation, although harmless at first glance, continue to be a major concern in sectors of human activity dealing with the fogging phenomenon. For example, the occurrence of surface fog lowers the energy conversion efficiency of solar cells[4,5] and reduces the amount of light transmitted through the greenhouse claddings.[6] The fogging of camera-guided instruments such as those employed during minimally invasive surgeries (e.g., laparoscopes, arthroscopes, etc.) slows down the normal progress of surgical procedures and puts the patient's life at risk, due to the low quality of images.[7,8] In the same vein, using fogged eyewear during physical activity can be both dangerous and incredibly frustrating.[9–11]

To prevent or avoid the effects of condensation, most of the antifogging strategies explored thus far have aimed either to change certain environmental parameters, such as temperature[12–15] and relative humidity,[16–19] or to change the morphology of water drops, especially by interaction with a (super)hydrophilic coating.[20–23] Although the first strategy has proven to be very effective in preventing the occurrence of surface fog, the fabrication of (super)hydrophilic coatings has attracted

much more attention, in view of the growing number of papers published on this topic in peer-reviewed journals over the last five years.[24]

Due to its “water-loving” characteristics, a (super)hydrophilic coating causes fog drops to spread across the surface to form a continuous or nearly continuous thin film of water. As a result, the incident light is able to transmit through it without being scattered and the effects of fogging are thus minimized.[25] Recent literature shows that these coatings can be prepared in three different ways. The first way involves depositing inorganic materials that are either intrinsically hydrophilic, such as SiO2, or superhydrophilic after exposure to ultraviolet light, such as TiO2, ZnO, SnO2, WO3, and V2O5.[26–28] The second way involves depositing polymers containing hydrophilic groups, such as hydroxyl,[29 –31] amino,[32] carboxyl/ester,[33,34] or sulfonic[35,36] groups. Finally, the third way involves depositing blends of hydrophilic polymers with nanosized materials, such as solid or mesoporous SiO2 nanoparticles.[37,38]

Unfortunately, most of the coating techniques employed thus far often encounter constraints related to excessively time-consuming protocols, and use of chemicals that are either toxic or detrimental to the environment.[30,31] To overcome these limitations, the application of atmospheric pressure dielectric barrier discharges (AP-DBDs) can be a promising antifogging technology, in view of the variety of SiOxCy:H coatings that can be fabricated in one step from siloxane precursors, such as hexamethyldisiloxane (HMDSO) or tetraethoxysilane, with rapid deposition rates and tunable wetting properties.[39–41] In addition, the use of atmospheric pressure DBDs represents an eco-friendly and cost-effective alternative to conventional deposition techniques, such as dip-coating or layer-by-layer deposition, as neither solvents nor considerable amounts of siloxane precursors are needed for the fabrication of thin films.

In this regard, a very recent study conducted by Laroche et al.[42] in which several siloxane precursors (i.e., TMCTS, OMCTS, TMDSO, and HMDSO) were used for the fabrication of antifogging coatings by AP-DBD, revealed that 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) was the most promising and suitable siloxane precursor for that purpose. Although it was argued that the presence of Si–H bonds in the TMCTS, coupled with its cyclic structure, was behind the antifogging performance of plasma-deposited coatings, neither the kinetics of coating deposition nor the effect of gas composition on the antifogging performance was investigated.

Considering the above, the aim of this paper is to prepare SiOxCy:H coatings on glass samples using an AP-DBD under a controlled N2 gas atmosphere from different TMCTS and N2O concentrations to understand how the [TMCTS]/[N2O] ratio and the [TMCTS] + [N2O] sum affect the antifogging characteristics. The composition and structure of the coatings were investigated using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Surface roughness and coating thickness were measured using atomic force microscopy (AFM) and profilometry, respectively. The wetting behavior of the coated glasses was assessed by the sessile drop method and the antifogging performance was evaluated by visual inspection after exposure to hot water at 80°C and by the protocol described in the ASTM F 659-06 standard.

2 EXPERIMENTAL SECTION

2.1 Materials and sample preparation

The siloxane precursor used in this study was 1,3,5,7-tetramethylcyclotetrasiloxane (O4Si4(CH3)4H4), also known as “TMCTS” (Figure 1). Liquid TMCTS (NMR grade, purity ≥99.5%) was purchased from Sigma-Aldrich. Nitrogen (N2; grade 4.8) and nitrous oxide (N2O; 99.998%) were provided by Linde (Québec, QC, Canada). Acetone and methanol were purchased from Laboratories MAT (Québec, QC, Canada) and Commercial Alcohols (Ontario, ON, Canada), respectively. Rectangular-shaped glass samples (13 cm × 5 cm × 2 mm) were kindly provided by Multiver Ltd (Québec, QC, Canada). Before coating deposition, glass samples were ultrasonically cleaned with acetone for 10 min and rinsed with methanol and deionized water to remove any organic remnant.

Afterward, the glass samples were ultrasonically washed with deionized water for 10 min and wiped dry with a cotton cloth (Amplitude Kappa™; Contec, Inc., Spartanburg, SC).

Figure 1 Molecular structure of TMCTS (1,3,5,7-tetramethylcyclotetrasiloxane)

2.2 Deposition conditions

A setup such as the one shown in Figure 2 was used to prepare TMCTS-based coatings. The DBD reactor is composed of two parallel plate electrodes: the upper one is a 0.64-mm-thick alumina sheet coated with a conductive paint (3.5 × 3.0 cm), whereas the bottom one is a stainless- steel plate (13 × 9 cm). The plasma was generated in the interelectrode space upon application of a sinusoidal voltage with a peak-to-peak amplitude of 14 kV and a frequency of 3 kHz. These operating conditions correspond to the power dissipated in the discharge of 0.25 W/cm2.

The gas inlet consists of two independent lines, one for N2 (carrier gas, 6 L/min) and other for N2O (oxidant, variable flow rate). Unlike O2 or air, N2O makes it possible to obtain a homogenous discharge (Townsend discharge) for a wider range of concentrations (up to 1,200 ppm of N2O vs. up to 500 ppm of O2); therefore, a transient homogeneous-to-filamentary transition is less likely to occur.[43] To ensure a laminar flow during the deposition process, N2 and N2O were introduced into the interelectrode space after having passed through a diffuser. The flow rates of N2 and N2O were measured using mass flow controllers (Bronkhorst™; Ruurlo, Holland). A suspension of TMCTS droplets in N2 was injected into the interelectrode space at 1 L/min using a syringe pump (Fisherbrand™; Thermo Fisher Scientific, Runcorn, Cheshire, UK) coupled to a nebulizer (Mira Mist CE™; Burgener Research Inc., Mississauga, ON, Canada). Before the deposition process, glass samples were placed on the bottom electrode keeping the interelectrode distance at 1 mm. Afterward, the reactor chamber was pumped down to 10−2 Torr and filled with N2 until the pressure reached 760 Torr (1 atm). The deposition time was set at 20 min. Table 1 summarizes the deposition parameters that were varied in this study.

Although the chemistry inside the plasma is complex, in view of the variety of species that can be generated, such as OH, CO, or CH3, it has been deemed appropriate to propose the following reaction between the TMCTS and the N2O as a point of reference to set the [N2O]/ [TMCTS] ratios at 10 and 30:

O4Si4 (CH3)4H4 + 20 N2O → 20 N2 + 4 CO2 + 8 H2O + 4 SiO2. (1)

Figure 2 Dielectrics barrier discharges reactor used for the deposition of TMCTS (1,3,5,7-tetramethylcyclotetrasiloxane) – based coatings

2.3 Coating characterization

2.3.1 Stylus profilometry

Coating thickness was measured using a stylus profilometer. To this end, coatings were slightly scratched off with a tweezers tip each 2 mm from the entrance to the exit of the discharge. The average height of the resulting steps was measured in triplicate from the entrance and parallel to gas flow using a DektakXT™ profilometer (Bruker Nano Surface Division, Tucson, AZ) with a stylus force of 1 mg (radius = 5.0 mm). The uncertainty in the average thickness was calculated as the standard deviation of these measurements. Measurements lasting 60 s were performed at a distance of 500 µm. The deposition rate of the coatings was calculated by dividing the as- measured thickness by the deposition time.

Table 1 Deposition parameters

2.3.2 Fourier transformed infrared spectroscopy

The chemical bonds and structure of the coatings were analyzed using an FTIR spectrometer (Cary 660 FTIR; Agilent Technologies, Victoria, Australia) equipped with a DLaTGS detector and a Split-Pea attachment (Harrick Scientific Products, Pleasantville, NY). IR spectra were averaged over 128 scans and recorded from 400 to 4,000 cm−1 with a resolution of 4 cm−1. Origin software (Origin Lab Corp., v. 8.5) was used to fit Gaussian curves to overlapping bands for semi-quantitative analysis. For the sake of comparison, all spectra were baseline corrected and normalized with respect to the Si–O–Si asymmetric stretching vibration (i.e., νa Si–O–Si) at 1,000–1,200 cm−1. For each coating, spectra were recorded in triplicate at a distance of 0.5 cm from the entrance to the discharge. For each coating condition, the recorded spectra showed the same bands.

2.3.3 X-ray photoelectron spectroscopy

The surface composition of the coatings was investigated using a PHI 5600-ci spectrometer (Physical Electronics, Chanhassen, MN). A standard aluminum X-ray source (Al Kα1,486.6 eV) was used to acquire the survey spectra of Si, C, O, and N in the range 0–1,400 eV, whereas a standard magnesium X-ray source (Mg Kα1,253.6 eV) was used to acquire the high-resolution spectra of C and Si. Photoelectrons coming from an area of 5 × 10−3cm2 were detected at 45° with respect to the surface normal. XPS analyses were conducted at <10−8 Torr with no need for surface charge neutralization. The spectrometer work function was adjusted by setting the binding energy of the C–C/C–H feature at 285 eV. By least-squares minimization, curve fitting of the C1s and Si2p envelopes was conducted using Gaussian–Lorentzian functions and a Shirley-type background. Photoemission peak areas were calculated using PHI MultiPak™ software, v. 9.3. Nine analyses per sample were performed to assess the chemical homogeneity of the coatings—from the entrance to the exit of the discharge—and provide a mean value with its corresponding standard deviation.

2.3.4 Atomic force microscopy

The surface topography of the coatings was examined at the nanometer scale on 2 × 2 μm2 and 20 × 20 μm2 areas using an atomic force microscope (Dimension 3100 Veeco Digital Instruments by Bruker, Santa Barbara, CA) equipped with an etched silicon tip with a radius of curvature <10 nm (OTESPA probe; Bruker Nano Surface Division, Santa Barbara, CA). AFM images were acquired in the tapping mode at a scan rate of 0.5 Hz and a line resolution of 256 × 256. Two roughness parameters, namely the root mean square roughness (Rrms) and the mean roughness (Ra), were measured after image flattening (NanoScope Analysis software, v. 1.5 by Bruker).

2.3.5 Assessment of the wetting behavior

The wetting behavior of the coatings was assessed using the sessile drop method. Briefly, 3-μl water drops were dropped from a height of 1 cm and contact angles were measured after the pinning of the three-phase contact line using a Video Contact Angle System (VCA-2500 XETM; AST Products Inc., Billerica, MA). For each coating, nine droplets were deposited on different locations from the entrance to the exit of the discharge. The water contact angles (WCA) reported here are the average of the values measured on the right and left sides of the sessile drops.

2.3.6 Assessment of the antifogging performance

The antifogging performance of coated glasses was assessed using a fog quantification box, which was fabricated in accordance with the American Society for Testing and Materials (ASTM) F659-06 standard.[31] Briefly, coated glasses were placed over a bath containing water at 50°C to measure the “two-pass” transmittance of a 590-nm light as a function of time. According to the ASTM F659-06 standard,[44] if the percentage of light transmitted through the sample is ≥80% after 30 s of exposure to water vapor, the antifogging requirement is met.

Figure 3 Deposition rate as a function of the position for coatings deposited under different [N2O]/ [TMCTS] ratios and [N2O] + [TMCTS] sums. TMCTS, 1,3,5,7-tetramethylcyclotetrasiloxane

3 RESULTS AND DISCUSSION

3.1 Deposition rate of the coatings

Figure 3 shows the deposition rate of the coatings as a function of the position along the length of the discharge. The deposition profiles appear to be governed by the [N2O]/[TMCTS] ratio, in contrast to the deposition rates, which were, regardless of the deposition conditions, at their highest at∼2 mm from the entrance to the discharge. In this regard, two families of coatings can be distinguished from each other: those fabricated under a substoichiometric [N2O]/[TMCTS] ratio (R=10) and those fabricated under an overstoichiometric [N2O]/[TMCTS] ratio (R= 30).

In the case of substoichiometric conditions, deposition profiles were characterized by a deposition rate at the entrance of the discharge of a few nm/min, followed by a marked increase to 348 nm/min for the coating D and 336 nm/min for the coating A. Deposition rates were then shown to decrease steadily along the length of the discharge to∼10 nm/min at the exit. As regards the overstoichiometric conditions, deposition profiles showed similar features; although, deposition rates were significantly low compared to the substoichiometric case. Here, deposition rates in-creased less abruptly to 67.6 nm/min for the coating B and 98.3 nm/min for the coating C and then decreased more gently to 11 nm/min at the exit of the discharge. In addition, these deposition profiles suggest that the consumption of the precursor along the length of the discharge is slow compared to that of the samples A and D. Interestingly, for the same [N2O]/[TMCTS] ratio, the deposition profiles were almost superimposable.

These singularities aside, the trend observed in the deposition rates should not be surprising, as the amount of TMCTS must decrease along the length of the discharge as a result of interactions with plasma species (e.g., electrons, radicals, N2 metastables, etc.). In investigating SiO2-like coatings deposited from N2/N2O/HMDSO mixtures using an AP-DBD, Enache et al.[45] noticed similar deposition profiles. As did Premkumaret al.,[46] who prepared coatings from Ar/N2/O2/HMDSO mixtures, these authors formulated a simple model to explain the deposition profiles built on two basic assumptions. First, that the HMDSO molecules do not interact directly with the substrate; and

second, that the Si–O–Si-containing radicals resulting from the interaction of HMDSO molecules with the N2 (A3Σ+𝑢) species have sticking coefficients of one. Given that no coating was observed outside the discharge (>3.0 𝑐𝑚), it is reasonable to assume that the residence time of the fragments resulting from these interactions is greater than the time required for them to diffuse toward the glass surface.

3.2 Structural analysis of the coatings

Understanding the chemistry and structure of the coatings is key to explaining the antifogging performance of the coated glasses. For this reason, their IR spectra have been compared with that of the TMCTS and discussed further below, in terms of band broadening and shifts.

3.2.1 IR spectrum of TMCTS

Figure 4 shows a detailed assignment of the infrared bands of TMCTS. Vibrational modes of methyl groups resulted in two small bands, one at 2,968 cm−1 due to the C–

H asymmetric stretching (νa), and other at 2,922 cm−1 due to the C–H symmetric stretching (νs). Asymmetric (δa) and symmetric (δs) bending vibrations of C–H in CH3 groups were found at 1,406 and 1,259 cm−1, respectively.[47–51] As in the case of hydrocarbon compounds, these vibration modes are expected to occur at 1,450 and 1,380 cm−1, respectively; however, the silicon atom bonded to the methyl group causes them to shift to lower frequencies.[48–51] The absorption arising from the CH3 symmetric bending (δsCH3) is one of the most characteristic bands of the methylsilyl-containing siloxanes. The frequency at which this band is observed depends on the number of methyl groups bonded to the silicon atom in the (CH3)nSiO(4−n)/2 units (where n=1, 2, and 3). In general, this band occurs at∼1,275 cm−1 when n=1 (“T-units”), at∼1,265 cm when n=2 (“D-units”), and at∼1,255 cm−1 when n=3 (“M-units”).[52–54] This absorption is always accompanied by the Si–C stretching (ν) and CH3 rocking (ρ) vibrations. In M-units, both the CH3 rocking and the Si–C asymmetric stretching occur at 845 cm−1, accompanied by Si–C symmetric stretching at 760 cm−1.[50,55] In D-units, the CH3 rocking appears at 855 cm−1, whereas the asymmetric and symmetric stretching vibrations of Si–C appear at 800 and 690 cm−1, respectively.[50,53,55] In T-units, only one peak in the 750–780 cm−1 range is observed, as in the case of TMCTS (vSi–C/ρ CH3 at 771 cm−1).[49,50]

FIGURE 4 Infrared spectrum of TMCTS (1,3,5,7-tetramethylcyclotetrasiloxane). a, asymmetric; s, symmetric; ν, stretching; δ, bending; ρ, rocking

Depending on the structure of the siloxane precursor, the frequency at which the Si–O–Si asymmetric stretching (νa Si–O–Si) occurs can vary from 1,010 to 1,125 cm−1. In linear siloxanes, this band broadens as the chain length increases and splits into two features at∼1,080 and 1,020 cm−1, when the number of Si–O–Si groups is ≥3.[56–59] In cyclic siloxanes, this band shifts from 1,020 to 1,090 cm−1 as the ring increases in size. As in linear siloxanes, this band broadens and splits into two features at∼1,050 and1,090 cm−1, but in this case, when the number of Si–O–Si groups is ≥6.[56–59] Considering the above, vibrational modes of Si–O–Si groups in TMCTS result in two bands, one at 1,074 cm−1 [49,51] due to the asymmetric stretching and other at 575 cm−1 due to the symmetric stretching.[47,51,55]

The band at 2,167 cm−1 can be assigned to the Si–H stretching (νSi–H).[49,51] The position of this absorption, which usually ranges from 2,100 to 2,250 cm−1, is sensitive to the chemical environment of the Si–H group.[60–62] In addition, the ring strain in cyclic siloxanes and the number of O–Si–O units in the vicinity of the Si–H group also influence the position of this vibrational mode. For example, in cyclic siloxanes [(CH3)HSiO]n (n=3–7), the Si–H stretching shifts to high frequencies as the cycle decreases in size. The electron attraction from adjacent O–Si–O units also causes ν Si–H to shift to high frequencies as a result of the Si–H bond strengthening.[60, 61]

In the IR spectrum of TMCTS, the ν Si–H band is accompanied by another, more intense band at 873 cm−1,which can be attributed to the asymmetric bending of H–Si–O groups (δa H–Si–O).[49]

3.2.2 IR spectra of the coatings

Figure 5 shows the IR spectra of the coatings deposited under a substoichiometric ratio (coating A, [N2O]/[TMCTS] = 10) and under an overstoichiometric ratio (coating B, [N2O]/[TMCTS] = 30). These samples were selected because of their different antifogging performances (Section 3.5).

Coatings deposited under a [N2O]/[TMCTS] = 10 Despite the addition of N2O (oxidant) in the discharge, bands related to methyl groups such as

νa,sCH3 in the 2,866–2,970 cm−1 range (Figure 5a), δaCH3 at 1,410 cm−1 (Figure 5c), δs CH3 at 1,274 cm−1,and ν Si–C/ρCH3 in the 750–850 cm−1 range (Figure 5d) were observed. The presence of these bands suggests that under a [N2O]/[TMCTS] ratio of 10, it is not possible to completely remove the methyl groups from the TMCTS during the deposition process. The absence of the Si–H-related bands, namely ν Si–H (Figure5b) and δa H–Si–O (Figure 5d), supports the high reactivity of the Si–H bond compared to that of Si–C or C–H bonds.[63] Accompanied by a shoulder at∼1,150 cm−1,the Si–O–Si asymmetric stretching (νa Si–O–S) broadened and occurred at a frequency lower than that observed in TMCTS (1,029 vs. 1,074 cm−1). The unresolved band between 3,000 and 3,650 cm−1 due to O–H stretching vibrations[50,54,64] (Figure 5a), substantiates the presence of silanol groups in the coatings, as revealed by Si–O bending in Si–OH groups at∼920 cm−1(Figure 5d).[50,54,65,66] Another less intense and complex band between 1,500 and 1,750 cm−1 can be attributed to the presence of C═O (ν C═O in aldehydes and amides) and N–H bonds (δN–H2)[64] in the coatings (Figure 5c).

Coatings deposited under a [N2O]/[TMCTS] = 30 Because of the increase in the N2O concentration in the discharge, the CH3-related absorptions,

such as ν Si–C/ρCH3 in the 750–850 cm−1 range (Figure5d), δa CH3 at 1,410 cm−1, and δs CH3 at ∼1,275 cm−1 were less intense than those observed in the coatings deposited under a [N2O]/[TMCTS] = 10. Some contribution to the band between 750 and 850 cm−1 may also come from the bending of Si–O–Si groups (δ Si–O–Si), which typically occurs at 800 cm−1 (Figure 5d).[66,67] The oxidative removal of CH3 groups was coupled with an increase in the intensity of the δ Si–OH band (∼920 cm−1) and the disappearance of the Si–H functionality. As in the case of coatings prepared

under a [N2O]/[TMCTS] = 10, the Si–O–Si asymmetric stretching broadened and occurred at a frequency lower than that observed in the precursor, accompanied by a shoulder at ∼1,170 cm−1 (Figure 5d). Considering the intensity enhancement of both the δ Si–OH band (Figure 5d) and the broad band ranging from 3,000 to 3,650 cm−1, it can be argued that the amount of silanol groups is greater than that found in coatings prepared under a [N2O]/[TMCTS] = 10 (Figure 5a). Bands related to carbonyl-and nitrogen-containing groups (i.e., C═O, NH) also occurred in the 1,500–1,750 cm−1 range, yet they were less intense than the ones observed in coatings prepared under a [N2O]/[TMCTS] = 10 (Figure 5c).

FIGURE 5 Infrared spectra of (a) 4,000–500 cm−1, (b) 3,800–2,400 cm−1, (c) 1,800–1,300 cm−1, and (d) 1,300–700 cm−1 regions of coatings deposited under a R=10 (in red) or R=30 (in blue)

3.2.3 Curve fitting of IR spectra

Along with the CH3 symmetric bending (δs CH3), the analysis of the band between 1,000 and 1,200 cm−1 (νa Si–O–Si) makes it possible to obtain the structural information of the coatings. It has been reported that this broad band can be curve fitted with two,[66] three,[49,68] or four [52] components. In the case of Si O2 coatings, the Si–O–Si asymmetric stretching results in two spectral features, one at ∼1,070 cm−1 due to the in-phase vibrations, known as “AS1”, and the other at ∼1,200 cm−1 due to the out-of-phase vibrations, known as “AS2.”[66,69,70] The ratio of the area below the AS2 band to the area below the AS1 band allows for the qualitative assessment of the structural disorder in the coating. Indeed, the higher the AS2/AS1 ratio, the more disordered the SiO2 structure is.[71]

According to Rouchon et al.[72] the structural disorder is due to the presence of voids, stress, suboxides, or roughness at the interface coating/substrate, among other defects.

In the case of carbon-containing SiOx coatings (i.e., SiOxCy:H), at least a third component must be considered when analyzing the νa Si–O–Si band. Of note is the elegant curve fitting conducted by Grill and Neumayer in SiOxCy:H coatings prepared using plasma-enhanced chemical vapor deposition from TMCTS.[49] These authors attributed the first component at 1,020-1,035 cm−1 to the presence in the coatings of cyclic siloxanes with D3h symmetry and silicon suboxidized states (e.g., O–Si–Si and O–Si–C). The second component at 1,065–1,070 cm−1 was attributed to a Si–O–Si network with a Si–O–Si angle ∼144° and some preserved TMCTS rings. Finally, the third component at 1,135-1,150 cm−1 was attributed to the presence of cage-like entities with Si–O–Si angles >144°. It was also suggested that some contribution to the Si–O–Si envelope comes from the asymmetric stretching of Si–O–C and C–O–C groups; that said, no additional curve fitting was carried out in this regard.

Curve fitting of the most representative vibrational modes of Si–O–X (X = Si, C, and H), Si–(CH3)n (where n = 1, 2, and 3), and CH3 groups was conducted considering the above. To illustrate this point, an example is given in Figure 6.

Table 2 summarizes the band areas and full width at half maximum of the bands of the coatings. If the [N2O]/[TMCTS] ratio remains unchanged, an increase in the amount of TMCTS and N2O in the discharge (i.e.,[N2O] + [TMCTS] sum) does not appear to significantly modify the chemistry and structure of the coatings. This result reveals that the chemistry of the coatings is mainly governed by the [N2O]/[TMCTS] ratio, and not by the [N2O] + [TMCTS] sum. For this reason, coatings obtained under a [N2O] + [TMCTS] = 100 and different [N2O] + [TMCTS] ratio (i.e., coating A (R = 10) and coating B (R= 30)) have been used as a reference in this study (Figure 6).

A detailed analysis of certain IR bands provides interesting structural information. For example, as regards the νa Si–O–Si absorption, the observed band asymmetry/broadening and shift to low frequencies when compared to that of the TMCTS can be due to a variation in the Si–O–Si bond angle.[75] Changes in the Si–O–Si bonding environment, which are not surprising as only 0.1 eV is required to decrease by 13° the Si–O–Si angle,[76] substantiate the presence in the coatings of various Si–O–Si arrangements, such as cyclic entities, chains, and cage-like structures. The presence of cage-like structures in the coatings supported by band in the 1,135–1,150 cm−1 range suggests that the (Si–O)4 rings of TMCTS were partially preserved. This band broadened and shifted to higher frequencies as the amount of N2O injected in the discharge increased, thus indicating the likely formation of cage-like structures with different symmetries, such as closed and open cages.[63] Furthermore, the ratio of the area below the “cage-like” band to that of the νa Si–O–Si band was greater in coatings prepared under a R = 30 (46% vs. 40%), intimating a higher proportion of these entities in the coatings.

Bands related to “open” structures were also found to vary with the [N2O]/[TMCTS] ratio. In both coatings, the FWHM of the “DH4 ring/SiO2-like” band was less than that reported in the SiO2 (∼75 cm−1), suggesting that the distribution of the Si–O–Si bond angles is less spread and relatively close to 144°.[75,77] The observed decrease in “O–Si–Si/O–Si–C/“D4H ring/SiO2-like” area ratio when [N2O]/[TMCTS] ratio went from 10 to 30, which may account for the shift in the νa Si–O–Si maximum from 1,029 (R = 10) to 1,050 cm−1 (R = 30), due to the oxidative removal of carbon from the O–Si–C entities. This result is consistent with the loss of the CH3 groups, as revealed by the decrease in the band due to the CH3 symmetric bending (νs CH3). The frequency at which the δs CH3 absorption occurred (∼1,275 cm−1) in both coatings indicates that most of the CH3 groups were forming “T” units (i.e., O3Si–(CH3)1); that said, its asymmetry provides evidence of the presence of “D” (i.e., O2Si–(CH3)2) and “M” (i.e., O1Si–(CH3)3) units in the coatings.

At low N2O/TMCTS ratios (R = 10), the Si–C bond was mainly preserved, in light of the bands at 1,275 cm−1 (δs CH3 in Si(CH3)n) and 774 cm−1 (νSi–C/ρCH3; Figure 5d). However, under these conditions, the emergence of the δSi–OH absorption at∼920 cm−1 was accompanied by the loss of the Si–H functionalities, as neither ν Si–H nor δH–Si–O vibrations were observed (Figure 5b).

Accordingly, one can argue that the formation of Si–OH groups at low oxidant/precursor ratios occurs primarily through the breaking of Si–H bonds, most likely upon interaction with the N2 (A3Σ#$) species[78] and/or with the oxygen atoms coming from the dissociation of the N2O in the plasma phase.[79] However, the hydroxylation of a small number of Si–CH3 groups, that is, the conversion of Si–CH3 groups into Si–OH ones cannot entirely be excluded. Infact, at high N2O/TMCTS ratios (R = 30), it is reasonable to assume that most of the Si–CH3 groups underwent hydroxylation, in light of the sharp decrease in the δs CH3 band con-current with the increase in the δSi–OH feature.[80]

FIGURE 6 Curve fitting of the 650–1,300 cm−1 region for a coating deposited at [N2O] + [TMCTS] = 100 and [N2O]/[TMCTS] = 10. The bands resulting from νs O–Si–C,[73,74] δ Si–O–Si, and ν Si–C/ρ CH3 vibrations were also considered to provide further detail on the chemistry of the coatings. TMCTS, 1,3,5,7-tetramethylcyclotetrasiloxane

TABLE 2 Full width at half maximum (FWHM) of bands and integrated band areas (A) for plasma-deposited coatings in the 650–1,300 cm−1 range

TABLE 3 Surface composition of the coatings as a function of the position

aMiddle: At 0.5 cm from the entrance to the discharge.

3.3 Surface composition of the coatings

Table 3 shows the surface composition of the coatings fabricated at R = 10 and R = 30 in terms of atomic percent (at%) of carbon, oxygen, and silicon, as well as O/Si and C/Si ratios.

The surface composition of both coatings did not change significantly from the entrance to the exit of the discharge, suggesting that the fragmentation of TMCTS occurring along the length of the discharge is barely affected by the [N2O]/[TMCTS] ratio. Nonetheless, when compared with the atomic composition of TMCTS, coatings contained much less carbon (≪33%) and more oxygen (≫33%). In coatings deposited at R = 30, the average content of carbon and oxygen at the surface was ∼9% and 66%, respectively. These values were ∼20% and 53%, respectively, in coatings deposited at R = 10. Not surprisingly, coating deposition conducted under higher oxidizing conditions (R = 30) resulted in coatings with less carbon, thus supporting the removal of methyl groups observed in the FTIR analyses. Interestingly, the average content of silicon appears to be unaffected by the presence of the oxidant N2O in the discharge, as it remains almost unchanged. This, coupled with a relatively low carbon percentage compared to that of the TMCTS, accounts for the lowest C/Si ratios found in the coatings.

The O/Si ratios were greater than that of the stoichiometric silica (O/Si = 2). The nonstoichiometricity of the SiOxCy:H coatings provides strong evidence of the incorporation of oxygen from the discharge and substantiates the presence of silanol groups in the coatings. The low O/Si values observed in coatings prepared at R = 10, compared to those of the coatings prepared at R = 30, reveal the role of N2O in removing carbon from the TMCTS molecules. A slight amount of nitrogen, most likely coming from the N2in the discharge,[81] was found in both coatings. The presence of nitrogen has also been reported in SiOxCy:H coatings made from HMDSO/N2O and SiH4/N2O mixtures using a DBD operated in N2 at atmospheric pressure.[67] Interestingly, a low content of N2O in the discharge correlated with an enhanced nitrogen incorporation.

Although XPS analyses were consistent with the FTIR results, it has been deemed appropriate to provide further detail on the carbon and silicon bonding environments. As regard carbon, C1s envelopes can be resolved into three or two bands (Figure7a,b). In both coatings, the feature at 285.0 eV reveals that most of the carbon in the coatings comes from C–H–and C–C-containing groups.[82,83] The bands at ∼286 and 287.5 eV can be attributed to the presence in the coatings of single-bonded C–O/C–N and double-bonded C═O/N-C═O containing species, respectively.[82,83] With the XPS survey scan and the HRXPS analyses in mind, it can be said that coatings fabricated under a [N2O]/[TMCTS] = 10 contain more carbon and that this carbon is more oxidized than that of the coatings fabricated under a [N2O]/[TMCTS] = 30.

According to O'Hare's[84] and Alexander's groups,[85] the Si2p envelopes can be composed of up to four peaks depending on the number of oxygen and carbon atoms bonded to the silicon, that is, (CH3)nSiO(4−n)/2 (where n = 0–3). The first peak at 101.5 eV is generally attributed to M-units (n = 3), the second at 102.1 eV to D-units (n = 2), the third at 102.8 eV to T-units (n = 1), and the last peak at 103.4–103.6 eV to Q-units (n= 0). As shown in Figure 7c,d, the Si2p core level can be

described by the above-mentioned components. Interestingly, the coating deposited under the overstoichiometric ratio (Figure 7d) is more oxidized, given that the Q-units + T-units/D-units + M-units ratio is greater than that found in the substoichiometric case (0.75 vs. 0.5; Figure7c). On this basis, it can be concluded that the coatings deposited at R = 30 possess a higher inorganic character and thus are somewhat closer to the SiO2-like structure. This view concurs with the results shown in Table 3. Indeed, the coatings prepared at R = 30 contain more oxygen but less carbon than those prepared at R = 10.

FIGURE 7 C1s and Si2p core level spectra of the coatings (a,c) A (R = 10) and (b,d) B (R = 30) on areas located at 0.5 cm from the entrance to the discharge

3.4 Surface topography of the coatings

Coatings were quite homogeneous and exhibited no surface defects at the microscale (Figure 8). Interestingly, an increase in the N2O in the discharge does not appear to affect significantly the morphology of the surface features nor the roughness (compare Figures 8a,b and 8c,d). Indeed, coatings were smooth, in view of the small Rrms values (2–4 nm) for both 2 × 2 and 20 × 20 μm2 areas. A similar trend was noticed in the mean roughness measured on both 2 × 2 and 20 × 20 μm2 areas, with Ra values of 2 and 1.8 nm, respectively. In either case, plasma-deposited coatings were rougher than the glass substrate, which was characterized by a surface roughness of 0.5–1.5 nm (not shown).

3.5 Antifogging performance and wetting behavior of the coatings

With regard to the antifogging performance of the coated glass, transmittance curves showed a marked decrease within the first seconds followed by a very slow recovery (Figure 9a).

According to Chevallier et al.,[31] the drop in light transmission observed within the first 5–10 s is due to the scattering of incident light provoked by water drops and can be correlated with the antifogging performance by an exponential relationship, as follows:

where T is the light transmission (%), t is the elapsed time(s), and is the time required for light transmission to decrease to a minimum value.

Although simple, Equation 2 proves successful in predicting whether a coated glass will be fogging-resistant or not, because small values of k are linked to a better antifogging performance. Indeed, the smaller the exponent k, the slower the drop in light transmission, and thus the more likely the coated glass to meet the antifogging requirement defined in the ASTM F659-06 standard, namely light transmittances above 80% after 30 s of exposure to water at 50°C (Table 4).

Fog testing revealed that uncoated glass fogged up promptly when exposed to water vapor at 50°C, whereas glasses coated with a TMCTS-based film maintained, in the worst case,∼66% of the light transmitted at 30 s under the same fogging conditions (coating D). Further, the analysis of light transmittance at 30 s makes it possible to distinguish two families of coatings in terms of the antifogging response: those that meet the antifogging requirements (coatings deposited under a [N2O]/[TMCTS] = 30) and those that did not (coatings deposited under a [N2O]/[TMCTS] = 10). Coatings deposited under high oxidizing conditions exhibited superior antifogging performance (transmittances >80% at 30 s) and featured a drop in light transmission faster than that observed in coatings prepared at low oxidant/precursor ratio (i.e., smaller values of k). Interestingly, the antifogging performance and the wetting behavior of the coatings can be correlated through the ratio, that is, the area below the IR band due to the symmetric bending of CH3 groups to the area below the IR band due to silanol groups. It was found that the smaller the ratio, the lower the contact angle, and thus, the better antifogging performance.

Numerous studies on wetting phenomena have demonstrated that the wetting behavior of any surface is not only governed by its chemistry but also by its roughness. According to the Wenzel model,[86] when a water droplet meets a rough surface, the resulting contact angle can be described as follows:

where θrough is the contact angle measured on a rough surface (i.e., apparent contact angle), Rf is the roughness factor, which is defined as the ratio between the actual surface area in contact with the liquid drop and its projection onto a planar surface (equivalent to a smooth surface), and θsmooth is the contact angle for an ideal smooth surface.

From Equation 3, it can be inferred that an increase in the roughness factor enhances the hydrophilicity of the coating (i.e., decrease in the water contact angle) if the water contact angle is below 90°. Nonetheless, the decrease in the WCA observed in the plasma-deposited coatings as the ratio increased from 10 to 30 cannot be due to arise in surface roughness, because Rrms and Ra values remained virtually unchanged. Accordingly, the antifogging performance of the coatings prepared at R = 30 can primarily be explained in terms of surface chemistry governed by the Si–OH groups. These hydrophilic groups cause fog drops to spread to form a thin film of water on the surface (WCA = 5°–10°), resulting in a non-fogged surface such as that shown in Figure 9b.

Numerous studies hold that superhydrophilic coatings are “unstable” in air, as they tend to reduce the solid/air interfacial energy as a result of the adsorption of airborne hydrocarbons/pollutants.[87] This natural phenomenon, or “surface aging” which is intrinsically linked to high energy surfaces, may disable the antifogging performance long term. The most recent

literature reveals that a reasonable approach to address this problem involves designing coatings that either reduce contaminant adsorption (e.g., fluorosurfactant-based coatings) or breakdown the adsorbed pollutants through suitable redox reactions (e.g., TiO2-based coatings) while enabling water drops to wet the surface.

Even though the coatings reported in this study do not fall within one of these categories, no significant degradation of the antifogging performance was observed after three or more antifogging tests. Before the assessment of the antifogging performance, coated glasses were immersed several times in distilled water at room temperature, as required in the ASTMF659-06 protocol. This result leads to the belief that atmospheric dust/pollutants adhered to the surface are washed away readily. Very recently, Poncin-Epaillard et al.[88] elegantly demonstrated that plasma technology can be used as a promising tool for the deposition of antifogging coatings endowed with self-cleaning features, without the need for previous immersion in water to perform. Codeposition of 1H, 1H, 2H-perfluoro-1-decene with either acrylic acid or 2-(dimethylamino)ethyl methacrylate showed interesting results in this regard, thus opening a door to a new family of antifogging coatings with oleophobic/hydrophilic characteristics.

FIGURE 8 Atomic force micrographs (AFM) of the plasma-deposited coatings on glass using a homogeneous N2/N2O Townsend discharge. (a) R = 10, (b) R = 30 on a 2×2μm2 scanning area, and (c) R = 10 and (d) R = 30 on a 20 × 20μm2 scanning area (AFM analyses were performed on areas located at 0.5 cm from the discharge)

FIGURE 9 (a) Percentage of light transmitted as a function of time through coated glasses fabricated under different [N2O]/[TMCTS] ratios and [N2O] + [TMCTS] sums. (b) Side view of a coated glass obtained at [N2O]/[TMCTS] = 30 and [N2O] + [TMCTS] = 100 when exposed to water vapor at 80°C (fog testing was performed on areas located at 0.5 cm from the discharge)

TABLE 4 Data obtained from the light transmission curves shown in Figure 9, water contact angles (WCA), and ratios (from Table 2)

4 CONCLUSIONS

A coating made from TMCTS with remarkable antifogging performance was prepared for the first time using a N2 atmospheric plasma containing minute amounts of nitrous oxide (N2O). The chemistry, surface topography, and antifogging performance of the coatings deposited under different conditions were studied in detail. Light transmission measurements (ASTM F 659-06) revealed that the antifogging performance was governed by the [N2O]/[TMCTS] ratio and not by the [N2O] + [TMCTS] sum. The better antifogging performance observed in glass substrates coated under generous oxidant conditions ([N2O]/[TMCTS] = 30), compared with those coated under less oxidant conditions ([N2O]/[TMCTS] = 10) was due to the ability of the coatings to spread fog drops to form a thin film of water on the surface (WCA =5°–10° vs. 55°–62°). Considering that the coatings deposited at [N2O]/[TMCTS] = 30 were very smooth (Rrms=2.4–2.6 nm), their superhydrophilicity, and thus, their antifogging characteristics are due to a surface chemistry characterized by small Si–CH3/Si–OH ratios. The lack of correlation between the antifogging performance and the [N2O] + [TMCTS] sum can be viewed as an advantage if the atmospheric pressure Townsend dis-charges are used to fabricate antifogging coatings on an industrial scale. In this regard, minimizing the total amount of gases injected into the discharge while keeping a suitable [N2O]/[TMCTS] ratio would undoubtedly reduce the manufacturing cost and lead to economic savings.

ACKNOWLEDGEMENTS

The authors thank Pascale Chevallier and Stephane Turgeon for their precious help and advice concerning XPS and FTIR analyses. AFM analyses were performed at the University of Toulouse. This study was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada (G. L.), PRIMA-Québec (G. L.), and the Centre Québécois sur les Matériaux Fonctionnels (CQMF; G. L.).

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