# Tunable plasma-polymerized organosilicon films for toluene sorption

Stéphane Boyer, Vincent Rouessac, Stéphanie Roualdes, Valérie Flaud, Diane

Rebiscoul, Sabine Devautour-vinot

► To cite this version:

Stéphane Boyer, Vincent Rouessac, Stéphanie Roualdes, Valérie Flaud, Diane Rebiscoul, et al. Tunable plasma-polymerized organosilicon films for toluene sorption. Plasma Processes and Polymers, 2025, pp.e70060. 10.1002/ppap.70060. hal-05176336

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Submitted on 22 Jul 2025

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RESEARCH ARTICLE

OPEN ACCESS

# Tunable Plasma-Polymerized Organosilicon Films for Toluene Sorption

Stéphane Boyer¹,² ID | Vincent Rouessac¹ | Stéphanie Roualdes¹ | Valérie Flaud² | Diane Rebiscoul³ | Sabine Devautour-Vinot² ID

¹Institut Européen des Membranes, IEM, UMR 5635, Univ Montpellier, ENSCM, CNRS, Montpellier, France | ²Institut Charles Gerhardt Montpellier, ICGM, UMR 5253-CNRS/UM/ENSCM, Pôle Chimie Balard Recherche, Montpellier, Cedex, France | ³University of Montpellier, CEA, CNRS, ENSCM, ICSM, Bagnols-sur-Cèze, Cedex, France

**Correspondence:** Vincent Rouessac (vincent.rouessac@umontpellier.fr) | Sabine Devautour-Vinot (sabine.devautour-vinot@umontpellier.fr)

**Received:** 17 April 2025 | **Revised:** 23 June 2025 | **Accepted:** 26 June 2025

**Funding:** We would like to thank Jim Cartier and Christophe Charmette for their technical assistance, Philippe Gaveau for NMR spectra acquisition and Bertrand Rebiere for SEM imaging. XPS and NMR experiments were performed with the support of the Balard Plateforme d'Analyses et de Caractérisation (PAC Balard). We acknowledge the support of LabUM Chimie at the University of Montpellier and the financial assistance provided by the French government through the National Research Agency (ANR) under the Investments for the Future program (Grant No. ANR-16-IDEX-0006).

**Keywords:** organosilicon film | PECVD | QCM | sorption | toluene

## ABSTRACT

Organosilicon polymers were deposited on Quartz Crystal Microbalance (QCM) supports using Plasma-Enhanced Chemical Vapor Deposition (PECVD) with a cyclic injector. By tuning plasma power and precursor flux, film structure was optimized for toluene sorption. A key outcome is the establishment of non-trivial correlations between the chemical structure and textural properties of the films and their toluene sorption behavior. Depending on these parameters, films exhibited either glassy or rubbery sorption profiles. Soft plasma conditions yielded rubbery-like films with high toluene sensitivity, full reversibility, and promising selectivity, remaining unaffected by water vapor. These findings demonstrate that fine control over PECVD conditions facilitates the rational design of high-performance sensing layers for selective VOC detection and enhanced Indoor Air Quality management.

## 1 | Introduction

The time spent in enclosed spaces makes Indoor Air Quality (IAQ) a major public health concern [1]. In living and working places, the pollutant levels are typically estimated to be 2–5 times greater than those in outdoor air, with even higher concentrations in industrial settings due to increased emission sources, confined spaces, prolonged exposure, and the use of specific industrial chemicals [2]. A wide array of hazardous pollutants is responsible for poor IAQ, including toxic gases (NOx, COx, SOx, ...) and a myriad of Volatile Organic Compounds (VOCs). Among indoor emitted VOCs, BTEX (Benzene, Toluene, Ethylbenzene, and Xylene) are particularly abundant and of major concern. BTEX have been listed as a priority

pollutant by the European Chemicals Agency (ECHA), which recommends short-term and long-term exposure limits (STEL and LTEL, respectively) to prevent workers and people from adverse health issues [3–5].

There is thus a critical need for low-cost, real-time BTEX sensors with high sensitivity, selectivity, accuracy, and minimal reagent consumption. In this context, chemical sensors that convert physicochemical changes into detectable physical responses, such as electrochemical, optical, magnetic, heat-sensitive, and mass-sensitive sensors, hold great promise. Among these technologies, Quartz Crystal Microbalances (QCMs) are highly competitive [6]. The integration of a sensitive and selective layer within QCM technology enables precise

<footnote>This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly
cited.</footnote>

<footnote>© 2025 The Author(s). *Plasma Processes and Polymers* published by Wiley-VCH GmbH.</footnote>
detection of volatile contaminants through changes in the
oscillation frequency of the quartz crystal. Such changes result
from the sensitive layer mass increase due to the adsorption of
contaminants [7]. The sensor performance is thus largely in-
fluenced by the physicochemical interaction between the coated
layer and the target contaminant, making the selection of the
sensitive layer, as the active material in QCM devices, critical.
In terms of sensitivity, the material affinity for the target con-
taminant should be carefully balanced to achieve significant
adsorption while ensuring reversible behavior. Selectivity is also
a key factor, as the sensing environment usually contains other
vapors. A hydrophobic layer is ideal to minimize the adsorption
of polar molecules, such as water vapor. The robustness of the
sensitive layer is also essential, as it must remain stable under
varying conditions. Additional factors, such as cost, film-
forming ability, low toxicity, rapid detection, and long-life effi-
ciency, should also be considered. Various conventional porous
solids have been envisaged for VOCs detection/sorption pur-
poses, including activated carbons, zeolites, Metal-Organic
Framesworks and metal oxide materials [8–13]. However,
these materials rarely fulfill all the requirements mentioned
previously. Alternatively, polymers represent a very competitive
class of materials for VOCs detection, including polyisobutylene
(PIB) and polybutadiene (PBD), or organosilicon (SiOCH)
polymers like polydimethylsiloxane (PDMS) and poly
(methylphenylsiloxane) [14–16]. Organosilicon polymers com-
bine high affinity toward typical VOCs with low toxicity,
good thermal and chemical stability, pronounced hydrophobic-
ity, and resistance to moisture. Consequently, they have
received remarkable attention for VOCs sensing applications,
including BTEX [17–19]. More specifically, organosilicon films
prepared by Plasma-Enhanced Chemical Vapor Deposition
(PECVD) have demonstrated a high affinity for apolar aromatic
VOCs, offering faster and reversible equilibration with sorbed
analytes compared to bulk polymers, due to their micrometer-
scale thickness [20]. Interestingly, the combination of PECVD-
deposited SiOCH films has been reported for the detection of
BTEX. By adjusting the deposition conditions and precursor
type, Nguyen et al. have prepared organosilicon films ranging
from polymer-like to ceramic-type by radio-frequency PECVD
and established preliminary correlations between films’ chem-
ical composition and BTEX sorption properties [21]. It has been
concluded that the sensitivity of the organosilicon film to BTEX
increases as the ratio between polymer-like bonds (Si-O-Si) over
end-chains (Si-(CH₃)ₙ and Si-OH) rises, consequent to the
softening of plasma conditions [22, 23]. Recently, some of us
have prepared a wide panel of organosilicon thin films from a
vapor of hexamethyldisiloxane (HMDSO) using microwave
PECVD (MW-PECVD) [24, 25]. The detection properties of
these films integrated to QCMs have been further analyzed
using toluene as BTEX, and ethanol or heptane as competitive
analytes. The results were in line with previous studies: the
films elaborated under soft plasma conditions, leading to ma-
terials with the strongest polymer character (characterized by
the highest Si-O-Si concentration and inter-chain free volume)
demonstrated the best sensitivity to toluene. Additionally, these
films exhibited superior selectivity, with a higher toluene up-
take over heptane and ethanol, compared to ceramic-type ma-
terials synthesized under more drastic plasma conditions.
Beyond organosilicon materials, various plasma polymers have
demonstrated significant potential in the adsorption of VOCs.

Nadekar et al. showed, through resistance measurements of plasma polymers, the selective sorption of polar molecules in iodine-doped plasma-polymerized thiophene and vinyl acetate (VAc) [26, 27]. Similarly, studies by Russell et al. and Zhang et al. analyzed the sorption behavior of VOCs in plasma-polymerized VAc and methyl methacrylate (MMA), employing models typically used for conventional polymers [28, 29]. These advances were made possible by combining PECVD techniques with QCM technology. Although these studies provide valuable insight into the relationship between the structural and textural properties of PECVD thin films and their sorption behavior, several critical points remain unresolved. Specifically, it means a deeper understanding of the correlation between the polymeric nature of organosilicon materials and their BTEX sorption and sensing properties. Addressing this challenge requires (i) refining PECVD conditions to precisely tailor the polymeric nature of organosilicon materials and (ii) conducting an in-depth analysis of toluene sorption mechanism using empirical models to optimize the sensing performance of these materials for BTEX detection.

In this study, polysiloxane films synthesized from HMDSO precursor are deposited on QCM supports using an innovative precursor cyclic injector. This approach aims to achieve higher precursor fluxes at specific stages of the deposition process during the “on” time of the cyclic injection, ultimately enabling lower overall plasma energy compared to traditional precursor introduction methods. The sorption profile of toluene displayed by the PECVD polymer-like organosilicon materials deposited on QCM supports is further described using empirical sorption models developed for conventional polymers, an area that remains scarcely explored to date [21, 28, 29]. A comprehensive correlation between PECVD deposition parameters, film structure and texture, and toluene sorption mechanisms is established to advance the development of PECVD films as BTEX detectors. The identification of non-trivial correlations between the structural and textural characteristics of PECVD-deposited organosilicon films and their toluene sorption behavior is a central contribution of our work. This correlation, derived from a combination of empirical sorption modeling and detailed material characterization, provides a deeper understanding of the physicochemical interactions at play, serving as a foundation for future sensing applications.

2 | Materials and Methods

2.1 | Plasma Deposits Synthesis

Organosilicon plasma deposits were synthesized in a 30 L capacitively-coupled plasma reactor manufactured by MECA2000 (Supporting Information: Figure S1). The vacuum system consisted of an ADIXEN PASCAL 2015 SD primary pump, completed with an ADIXEN ATP 80 turbo-molecular pump controlled with an ADIXEN ACT 200 T controller. The pressure in the chamber was monitored using a PENNINGVAC PTR 90 sensor and a LEYBOLD CAPACITRON DM21 controller. The two vertical parallel plate electrodes, spaced 2 cm apart, had a diameter of 10 cm. The plasma discharge was powered by a DRESSLER CESAR 136 radio-frequency (13.56 MHz) generator. Two different kinds of substrates
[AT-cut gold-covered quartz (Neyco) with a resonance frequency $f_0 = 6$ MHz used as QCM support and a 1.5 cm² piece of (100) p-type silicon wafer (Silicon Materials)] were affixed to the ground electrode using polyimide tape. The precursor, hexamethyldisiloxane (HMDSO, Sigma-Aldrich, NMR grade, >99.5%), was chosen due to its safety, cost-effectiveness, high chemical and thermal stability, and favorable deposition rate [30]. HMDSO was stored in a heated container (30°C) and transported to the deposition chamber through a stainless steel tube maintained at 50°C. Argon (Linde France, medical-chirurgical grade), kept at a pressure of 1.5 bar, served as the pushing gas. HMDSO was introduced into the reactor in the gaseous phase using a liquid injection system in conjunction with a vaporizer (KEMSTREAM VAPBOX 300) set to 80°C.

The baseline vacuum pressure within the plasma chamber before deposition ranged between $1.10^{-2}$ mbar and $5.10^{-2}$ mbar. Before vapor injection, both substrates underwent a surface pretreatment consisting of a 2-min exposure to a 50 W argon plasma at a working pressure of 0.1 mbar. Then the precursor injection and plasma power were successively initiated and maintained during the deposition process. Following deposition, the sample was maintained under vacuum for 10 min before retrieval.

The injector cyclically operated, controlled by both the frequency [0.1–3.5] Hz and the injection valve opening time [1–10] ms, directly impacting the precursor flux, which was assessed by the pressure variation $\Delta P$ measured before and after injector activation. The pulsed injection mode resulted in periodic pressure fluctuations, particularly noticeable at low frequencies. The representative $\Delta P$ value for each deposition was chosen as the peak pressure fluctuation. The selected injection parameters yielded $\Delta P$ values in the range of 0.08–1.45 mbar. The electrical discharge power $P_{ow}$ was set between 10 and 150 W. And the deposition time $\Delta t$ was adjusted between 1 and 60 min to achieve film thicknesses ranging from 0.1 to 3 µm.

In this study, the energetic character of the plasma discharge was quantified by the Plasma Power Parameter (PPP), defined as:

$$PPP = \frac{P_{ow}}{\Delta P \cdot M} \qquad (1)$$

$M$ being the molar mass of HMDSO (162.4 g.mol⁻¹). The PPP parameter expressed in W.mol·g⁻¹·mbar⁻¹ is a hybrid process parameter very close to the well-known Yasuda parameter [31], basically used to represent the specific energy input in the context of plasma polymerization. Although the Yasuda parameter traditionally uses the monomer flow rate to represent the quantity of matter in the active plasma zone, our study adopts the monomer partial pressure instead. This choice is justified by the fact that, unlike the cyclically fluctuating flow rate, the monomer partial pressure remains stable throughout the deposition process and is approximately proportional to the average flow rate. As a result, PPP is expressed in units of power, that is, instantaneous energy units, rather than in energy units as in the case of the Yasuda parameter. Nonetheless, PPP remains a valid and meaningful parameter to quantify the

monomer fragmentation rate in the active plasma zone, in the same spirit as the Yasuda parameter. PPP was ranging from 0.04 to 20 W.mol·g⁻¹·mbar⁻¹ so that plasma polymers from HMDSO with fine-tuned composition and properties could be obtained.

## 2.2 | Characterization

The morphology and thickness of the samples were analyzed using Scanning Electron Microscopy (SEM) and spectroscopic ellipsometry. SEM imaging was performed on a ZEISS sigma 300 instrument using cross-section deposits on both kinds of substrates, previously fractured to obtain side-view images. The error on a thickness measured by SEM is typically 10%–20%. Ellipsometric measurements were conducted on silicon wafer-deposited samples using a spectroscopic ellipsometer (Semilab GES5E) operating in the extended visible light range (250 nm –1 µm) at atmospheric pressure. Data acquisitions were performed from 1.25 to 3 eV, and thicknesses were extracted using the empirical Cauchy optical model with Winelli 2 software, achieving a goodness of fit exceeding 0.96. Both SEM and ellipsometry analyses were used to characterize all the samples generated for this study.

The chemical composition of the deposits was characterized using X-ray Photoelectron Spectroscopy (XPS) and ²⁹Si solid-state Nuclear Magnetic Resonance (NMR). XPS measurements were performed on silicon wafer-deposited samples using a Thermo Electron ESCALAB 250 instrument with a monochromatic Al Kα (1486.6 eV) X-ray source, analyzing a 500 µm diameter area. Photoelectron spectra were calibrated in terms of binding energy, referencing the O-Si component of oxygen at 532 eV for PDMS. To assess the influence of deposition parameters on chemical composition, 15 samples were analyzed, covering the entire range of PPP values explored in this study. Solid-state ²⁹Si NMR spectra were acquired using a VNMRS400 Varian spectrometer (400 MHz/9.4 T) with samples packed in 7.5 mm zirconia rotors. Spectra were recorded under Magic Angle Spinning (MAS) at 5 kHz, and relaxation time measurements were performed using the saturation recovery method. Single-pulse acquisition employed a π/6 pulse with a recycle delay of 2800 s. Due to the material quantity required for this analysis (30 mg per sample), which is impractical for plasma deposition given the extensive deposition time required, only two samples—whose plasma deposition conditions are provided later—were studied via NMR. Both XPS and NMR spectra were integrated to deduce chemical composition using the Avantage software.

The mass density of the deposits was determined using X-ray reflectometry (XRR) coupled with elemental chemical analysis. Measurements were carried out on a Bruker D8 diffractometer (Billerica, MA, USA) with Cu-Kα₁ radiation ($\lambda = 0.154$ nm). Standard symmetric Bragg–Brentano θ-2θ scans were used for data collection, covering the range 2θ [0°–4°] ($q_{max} = 0.2844$ Å⁻¹) with an angular step size of 0.002°. This analysis was carried out on four representative samples, whose deposition parameters will be detailed below. The electronic density ($\rho_e$, Å⁻³) of each deposit was determined by measuring the critical angle ($\theta_c$) and applying the following equation:
$$ \rho_e = \frac{\pi \theta_c^2}{\lambda^2 r_0} \qquad (2) $$

where $\lambda$ is the X-ray wavelength and $r_0$ is the electron's radius. Using $\rho_e$ and the elemental composition determined from XPS, the mass density ($\rho_m$, g.cm$^{-3}$) was then calculated according to:

$$ \rho_m = \frac{\rho_e \cdot \sum C_i A_i}{Na \cdot \sum C_i Z_i} \qquad (3) $$

where Na is the Avogadro number, $C_i$ is the atomic fraction of element i in the material, $A_i$ is the atomic mass of this element and $Z_i$ its atomic number.

The wettability of the plasma-deposited films was evaluated through water contact angle (WCA) measurements. Surfaces exhibiting good wetting by water (WCA < 90°) are classified as hydrophilic, whereas surfaces with poor wetting by water (WCA > 90°) are classified as hydrophobic. A lab-made experimental setup was used to deposit 6 µL of water droplets onto the sample surfaces. Contact angles were determined using Image J software, which was completed with the Drop Analysis plugin from EPFL. For each sample, six independent measurements were performed at three different locations, both right and left angles at each location, and the mean contact angle was reported along with the standard deviation.

## 2.3 | Vapor Sorption Response Testing

The vapor sorption response of the coated QCM supports was acquired using an Inficon STM2 thin-film oscillator monitor placed in a lab-made vacuum chamber [32]. The QCM was maintained at 25°C using a water-cooling system, and the chamber was initially vacuumed to 0.04 mbar. Sorption tests consisted in filling the chamber with the vapor of interest whose pressure (P) was gradually fixed at successive step values, first on an increasing scale from the initial vacuum up to the maximum pressure (sorption branch) and then on a decreasing scale from the maximum partial pressure down to

the initial vacuum (desorption branch). At each step, the quartz resonance frequency variation was recorded following a 2 min stabilization period. Both toluene, as BTEX, and water, as a competitive analyte, were used as vapours.

The measured quartz resonance frequency variation $\Delta f$ is the difference between the resonance frequency of the coated QCM support before and after vapor exposure. It is proportional to the mass uptake of the coated QCM support $\Delta m$ (g), that is, the mass of sorbate within the active film, using Sauerbrey's law:

$$ \Delta f = \frac{-2f_0^2}{A\sqrt{\rho_q \cdot \mu_q}} \Delta m \qquad (4) $$

where $f_0$ is the resonance frequency of the coated QCM support under initial vacuum, A is the active area of the deposited PECVD film on the quartz crystal (0.5 cm$^2$), $\rho_q$ is the quartz density (2.648 g.cm$^{-3}$), and $\mu_q$ is the shear modulus of the quartz crystal (2.947.10$^{11}$ g.cm$^{-1}$.s$^{-2}$) [33]. All parameters in Sauerbrey's equation remain constant except $\Delta f$ and $\Delta m$. The toluene and water sorption responses expressed as $-\Delta f/f_0$ versus P/P$_0$, where P$_0$ represents the saturated vapor pressure of the sorbate, are representative of the gravimetric sorption isotherms of the deposits.

# 3 | Results and Discussion

## 3.1 | Materials Morphology, Structure, and Texture

The SEM images shown in Figure 1 are cross-sections of a film deposited on both QCM support (Figure 1a) and silicon wafer (Figure 1b) with PPP and deposition time equal to 0.80 and 8 min, respectively. This film is morphologically representative of all those prepared in this study, as all exhibit a uniform, defect-free morphology and a dense structure, with no detectable porosity at the camera's resolution. The detachment of the film from the silicon support (Figure 1b) is not indicative of poor adhesion but rather a direct consequence of mechanical

<figure><img src="image_3.png" /><figcaption>FIGURE 1 | Cross-sectional SEM view of PECVD-organosilicon deposits on (a) a QCM support and (b) a silicon wafer (PPP = 0.80 and $\Delta t$ = 8 min).</figcaption></figure>
fracturing during sample preparation for SEM analysis. More-
over, the surface asperities observed on the deposit (Figure 1b)
are not inherent to the material but instead result from en-
vironmental dust particles or material/substrate fragments
generated during sample breakage for SEM observations.

Considering that deposition mechanisms can vary significantly
with the substrate surface properties [34], comparative SEM
measurements of film thickness were conducted on both the
QCM support and the silicon wafer. This comparative study, not
shown, revealed negligible differences in growth rates (GR)
between the substrates, with variations of approximately 10%,
that is, within the typical error margin for SEM thickness
measurements. This observation confirmed that the film's
thickness evaluated on a silicon wafer can reliably represent
that on QCM support. In the following, ellipsometry was pre-
ferred over SEM to evaluate the film thickness and the corre-
sponding GR.

Figure 2 shows the PPP dependence of the films GR. Under very
soft deposition conditions (PPP < 0.2), the material growth is
low, GR being around 10 nm · min⁻¹. As the deposition con-
ditions become more energetic, GR increases for PPP up to 1.
Beyond this PPP value, the evolution of GR is erratic in the
range 100–300 nm · min⁻¹. For PPP values below 0.2, the en-
ergy of the plasma is very low, leading to an excess of precursor
molecules that cannot be fragmented and are evacuated by the
vacuum system without contributing to the deposition. For
such low PPP values, only a small fraction of HMDSO frag-
ments take part in the polymerization process, leading to lim-
ited GR. For PPP values between 0.2 and 1, all precursor
molecules undergo fragmentation, preferentially at their rela-
tively weakest bonds, and participate in the polymerization
process. In this “soft regime,” GR is mainly influenced by the
number of precursor fragments involved in the deposition
process, which increases with the extent of precursor molecules’
fragmentation, which is directly related to the plasma input
power. Conversely, for PPP values exceeding 1.0, the plasma
energy exceeds the threshold required for complete

<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | Growth rate (GR) of PECVD deposits on silicon wafers as a function of PPP.</figcaption></figure>

polymerization, leading to over-fragmentation, i.e. the breaking
of all chemical bonds, which results in the formation of smaller
fragments as energy increases. In this “hard regime,” GR is
mainly governed by the size of precursor fragments involved in
the deposition process. Additionally, Figure 2 indicates that
deposit thickness is more easily controlled in the soft regime
(PPP < 1 corresponding to plasma input powers lower than
80 W) than in the hard regime (PPP > 1 for plasma input powers
higher than 80 W), for which the excess of energy complicates
the regulation of both the size and quantity of precursor frag-
ments. Supporting Information: Figure S2 supports the con-
clusion that the observed regime transition is primarily
triggered by the crossing of the PPP threshold around a value of
1, rather than by the absolute input power exceeding or falling
below 80 W. This further reinforces the relevance of PPP as a
representative parameter for the specific energy input.

Identifying these distinct deposition regimes facilitates the selection of representative samples for the subsequent analyses. Accordingly, four samples, labeled as PPP (X), where X represents the PPP value fixed to prepare the organosilicon film, were selected: (i) a sample having the minimum GR for a very low PPP value equal to 0.07 (yellow point in Figure 2 deposited at ΔP = 0.09 mbar and P<sub>ow</sub> = 10 W, GR = 10 nm.min<sup>-1</sup>), (ii) a sample prepared at a PPP value of 0.31 representative of the lower part of the soft regime (red point in Figure 2 deposited at ΔP = 0.02 mbar and P<sub>ow</sub> = 10 W, GR = 20 nm.min<sup>-1</sup>), (iii) a sample synthesized at a value of 0.80 at the frontier between the soft and the hard regime (brown point in Figure 2 deposited at ΔP = 0.09 mbar and P<sub>ow</sub> = 80 W, GR = 150 nm.min<sup>-1</sup>), and (iv) a sample prepared at a PPP value of 4.45 representative of average growth conditions in the hard regime (black point in Figure 2 deposited at ΔP = 0.02 mbar and P<sub>ow</sub> = 80 W, GR = 220 nm.min<sup>-1</sup>). In the following, these four samples will be labeled PPP (0.07), PPP (0.31), PPP (0.80), and PPP (4.45), the number in brackets referring to the PPP value in the preparation of films. They will be systematically associated with yellow, red, brown, and black, respectively.

XPS analysis was performed on 15 different samples, charac-
terized by PPP covering the range between the lowest and the
highest values encountered in this study, that is, 0.07 and 4.45,
respectively. In the survey spectra, the photoelectron peaks
Si2p, Si2s, C1s, and O1s are depicted (Supporting Information:
Figure S3) [35–37]. C/Si, O/Si, and O/C atomic percent ratios
calculated from XPS data are shown in Figure 3. (Figure 3a)
illustrates the evolution of the C/Si ratio with respect to PPP.
C/Si decreases from 3 to 2.1 as PPP value increases up to 0.2.
Beyond this point, it remains stable around 2.1, converging
toward the value observed for conventional PDMS [38]. The
initial decline suggests incomplete polymerization, as previ-
ously mentioned, wherein a significant proportion of silicon
atoms remain bonded to three carbon atoms, as in the HMDSO
precursor. Once the plasma discharge becomes sufficiently
energetic (PPP > 0.2), low-energy bonds, such as Si-C chain
ends, break, leading to complete polymerization. This primarily
results in siloxane chains with silicon atoms bonded to two
carbon atoms, similar to the atomic arrangement in conven-
tional PDMS. However, the C/Si ratio never reaches exactly 2,
likely due to the existence of residual -Si-[CH₃]₃ chain ends
and/or -Si[CH₃]₂-(CH₂)ₓ-Si[CH₃]₂⁻ bridges within the plasma
<figure><img src="image_5.png" /><figcaption>FIGURE 3 | (a) C/Si, (b) O/Si, and (c) O/C atomic percent ratios obtained from XPS analysis as a function of PPP.</figcaption></figure>

deposits or potential carbon contamination on the films surface [39, 40].

Further evidence of plasma polymerization yielding PDMS-like material is reflected by the evolution of O/Si proportion with PPP (Figure 3b), which remains close to 1 across all PPP values, whereas it is equal to 0.5 in HMDSO. A transition between these two values might have been expected for PPP values below 0.2, related to incomplete polymerization based on C/Si values. However, no noticeable transition is observed, likely due to residual O₂ and H₂O in the plasma reactor leading to oxidation, even in very soft plasma conditions, or due to superficial oxidation of plasma films upon exposure to air before XPS analysis.

In parallel, the value of the O/C ratio increases from 0.38 to 0.50 with the PPP increase (Figure 3c). While this trend indicates that the deposited materials progressively shift toward a PDMS-like composition (O/C = 0.5) as PPP increases, a higher O/C ratio is also expected to reduce the hydrophobicity of the material.

To characterize the chemical composition of the deposited films more deeply, deconvolution of the Si2p silicon peak (Supporting Information: Figure S4) made possible the identification of five distinct silicon coordination environments: S (SiC₄), M (SiOC₃), D (SiO₂C₂), T (SiO₃C), and Q (SiO₄). The proportion of the Si environments with respect to PPP does not show any clear correlation (Figure 4 and Supporting Information: Figure S5). However, M and D environments represent the most prevalent configurations across all samples, with average proportions of 38% and 28%, respectively. The high proportion of D environments supports the similarity between the plasma-polymerized films and conventional PDMS. The relatively high proportion of M environments, along with its noticeable decrease with PPP up to PPP = 0.2 before stabilizing around 35%, is attributed to: (i) the close structural resemblance to HMDSO in very soft plasma conditions in relation to incomplete polymerization, and (ii) the presence of Si-[CH₃]₃ chain ends, -Si[CH₃]₂-(CH₂)x-Si[CH₃]₂- bridges and/or surface carbon contamination, which makes the interpretation of XPS data challenging [40].

The challenges in interpreting XPS measurements, due to potential surface modifications caused by environmental contamination, were already highlighted by Dakroub et al. [25]. This underscores the need for complementary bulk analysis. Accordingly, solid-state NMR analysis was conducted as a

<figure><img src="image_6.png" /><figcaption>FIGURE 4 | Evolution of the proportion of silicon environments (S, M, D, T, and Q) in organosilicon thin films as a function of PPP. The vertical lines represent PPP values for samples PPP (0.07), PPP (0.31), PPP (0.80), and PPP (4.45).</figcaption></figure>

complementary chemical analysis to further validate and potentially enrich the conclusions derived from the XPS investigation. However, obtaining a sufficient amount of material for solid-state NMR using the PECVD conditions described in this study is problematic. The deposition rate for PPP (0.07) is too low to provide an adequate amount of material, while the PECVD parameters for PPP (4.45) are too intense for prolonged use without damaging the plasma reactor. Therefore, only both samples PPP (0.31) and PPP (0.80) synthesized with the intermediate PPP values were analyzed by solid-state NMR (Supporting Information: Figure S6).

Figure 5 illustrates the distribution of the five distinct silicon coordination environments (S, M, D, T, and Q) as determined by solid-state NMR analysis for PPP (0.31) and PPP (0.80). The M and D configurations are the most predominant for both samples, as previously highlighted by XPS. These environments represent 48% and 39% for PPP (0.31), and 34% and 46% for PPP (0.80), respectively. Compared to PPP (0.31), PPP (0.80) shows a higher proportion of D (46% vs. 39%) and T environments (12% vs. 6%), coupled with a decrease in M configuration (34% vs. 48%). These results might indicate that more intense deposition conditions result in greater precursor fragmentation, leading to a reduction in the relative abundance of M environments
<figure><img src="image_7.png" /><figcaption>FIGURE 5 | Proportion of silicon environments (S, M, D, T, and Q) in organosilicon thin films, as determined by solid-state NMR (filled bars) and XPS (hollow bars) for PPP (0.31) (red) and PPP (0.80) (brown).</figcaption></figure>

originating from HMDSO, to the benefit of other types of environments (S, D, and T). The Q (SiO₄) environment, previously observed by XPS, could not be detected by solid-state NMR. Moreover, the S environment and, to a lesser extent, the T environment, have noticeably lower proportions by NMR when compared to XPS. These discrepancies could be attributed to the fundamental differences in the two techniques: NMR provides bulk structural information, while XPS is a surface-sensitive method. Thus, the higher proportion of “extreme” environments, that is, having the highest carbon substitution rate (S) or the highest oxygen substitution rate (Q, and, to a lesser extent, T), detected by XPS may therefore be a consequence of carbon contamination or surface oxidation [37].

As a complement to the chemical composition, it is relevant to get an interest in the textural properties of the prepared organosilicon films. Indeed, both chemical and textural properties should have an influence on the vapor sorption properties of the materials. As a pertinent textural property, the mass density enables the accounting for the cross-linking degree of the material. In this study, mass densities were obtained through XRR from experimental determination of electronic densities (Supporting Information: Figure S7) and consideration of chemical composition from XPS. As the electronic density is a bulk property, it should have been more relevant to use bulk chemical composition instead of surface chemical composition from XPS, especially as it has been previously demonstrated that the surface of our deposits may be affected by oxidation and/or atmospheric carbon contamination. However, even if such surface modifications accounted for 10% of the total carbon or oxygen content (which is really an extreme situation [41]), the calculated value of mass density obtained using Equation 3 would only change by less than 4.10⁻⁴ g.cm⁻³. This is because the mass density is primarily influenced by the electronic densities in Equation 2. Therefore, any bias in the chemical composition would not significantly alter the mass density value.

Mass densities calculated from the critical angle measured from reflectivity data (Supporting Information: Figure S7) for PPP (0.07), PPP (0.31), PPP (0.80), and PPP (4.45) are 1.14, 1.26, 1.29, and 1.44 g.cm⁻³, respectively. These values are significantly higher than the density of the conventional PDMS (0.97 g.cm⁻³), highlighting another distinction between plasma-polymerized

films and conventional polymers. Indeed, materials prepared by plasma polymerization are intrinsically more highly cross-linked and thus denser than PDMS prepared by conventional methods [42]. This mass density increase follows the expected trend: as the plasma energy increases (PPP rises), precursor fragmentation is intensified, resulting in highly fragmented species whose recombination promotes cross-linking within the polymer network. This results in a denser material whose shorter and tighter chains hinder their motion, leading to a glassier polymer.

WCA measurements were performed to investigate the influence of PPP on the hydrophobic or hydrophilic nature of the films. The contact angles observed for PPP (0.07), PPP (0.31), PPP (0.80), and PPP (4.45) were 107°, 102°, 98°, and 95°, respectively. This obvious decreasing trend in WCA with increasing plasma energy (Supporting Information: Figure S8) is in accordance with the increase in the O/C ratio determined by XPS analysis. However, all samples exhibited WCA values above 90°, confirming their hydrophobic character, which is in agreement with previous studies and literature reports on organosilicon plasma polymers [25, 43].

## 3.2 | Materials Vapor Sorption Properties

Sorption properties of PPP (0.07), PPP (0.31), PPP (0.80), and PPP (4.45) deposits on QCM substrates were further investigated by monitoring the relative frequency shift, −Δf/f₀, of the devices exposed to pure toluene vapor (Figure 6). The flat and negligible signal of the bare QCM demonstrates that the plasma-polymerized film is responsible for the device response (Supporting Information: Figure S9). The resulting sorption isotherms exhibit profiles that are typical of polymeric materials, although they show noticeable differences depending on the deposition conditions of the organosilicon film. Specifically, these differences are linked to the rubbery/glassy nature of the polymer matrix, which governs the toluene/polymer interaction.

The sorption mechanism of toluene in the plasma-polymerized materials was further elucidated by analyzing the sorption profiles using models established for conventional polymers, that is, Henry's Law, Dual-Mode, and Flory-Huggins models.

For rubbery polymers, that is, polymers with chains that are relatively spread and flexible [41], the sorption process is regarded as a simple dissolution of the sorbate in the elastomer matrix. The concentration of molecules adsorbed by the polymer linearly increases with the sorbate pressure according to the Henry's Law [44, 45]:

$$C_D = k_D \times P \qquad (5)$$

where C_D is the concentration of sorbed molecules (cm³ [STP].cm⁻³ [polymer]), k_D the reflecting's constant, that is, the solubility coefficient (cm³ [STP].cm⁻³ [polymer].atm⁻¹) and P the vapor pressure (atm).

For glassy polymers, the Dual-Mode model is privileged [46–48]. In addition to the sorbate dissolution in equilibrium
<figure><img src="image_8.png" /><figcaption>FIGURE 6 | Relative frequency shift of QCM substrates coated with (a) PPP (0.07), (b) PPP (0.31), (c) PPP (0.80), and (d) PPP (4.45) as a function of increasing (filled symbols) and decreasing (empty symbols) partial pressure of toluene (squares) and water (stars), recorded at 298 K. The lines are guides for the eye.</figcaption></figure>

regions of the polymer according to Henry's Law, the filling of “microvoids,” due to the excess free volume reflecting the non-equilibrium nature of the glassy polymer, is taken into account by a Langmuir-type sorption component, which prevails at low pressure. The Dual-Mode model is thus expressed as follows:

$$C = C_D + C_H = P \times \left( k_D + \frac{C'_H \times b}{1 + b \times P} \right) \quad (6)$$

where C is the total concentration of dissolved molecules in the polymer (cm³ [STP].cm⁻³ [polymer]), combining the Henry's law component (C_D, Eq. 5) and the concentration of molecules trapped in “microvoids” (C_H): C'_H denotes the sorbate concentration in the “microvoids” (cm³ [STP].cm⁻³ [polymer]), and b is the Langmuir affinity constant of the sorbate in the “microvoids” (atm⁻¹).

At high pressure, deviation from Henry's Law is usually observed in the shape of the sorption isotherm, which displays a convex curvature to the pressure axis with the penetrant pressure. This behavior, typical of rubbery polymers, reflects

polymer swelling due to the high concentration of adsorbed molecules, which results from the clustering of sorbates. This plasticization phenomenon is described by the Flory-Huggins equation [49]:

$$\ln\left(\frac{P}{P_0}\right) = \ln(\nu_s) + (1 - \nu_s) + \chi(1 - \nu_s)^2 \quad (7)$$

where $\nu_s$ represents the volume fraction of the sorbate in the polymer matrix, and $\chi$ reflects the sorbate/polymer interaction energy.

As a first approximation, Henry's model was applied to the adsorption branch of toluene for PPP (0.07), PPP (0.31), PPP (0.80), and PPP (4.45) films deposited on QCMs. This model provides a reasonable fit to the experimental data within the intermediate pressure range, i.e. for toluene relative pressures between 0.2 and 0.6 (Supporting Information: Figure S10). The corresponding Henry's constant, k_D, decreases exponentially with the increase of PPP, reflecting the reduced toluene uptake, which can be attributed to both the increased density and the
higher O/C ratio of the plasma-polymerized films (Supporting Information: Figure S11). Notably, significant deviations from Henry's model are observed at both low and high pressures, suggesting that the Dual-Mode or Flory-Huggins models should be considered more appropriately. Accordingly, the plasma-polymerized films were classified into two distinct families. For the denser plasma polymers, PPP (4.45) and PPP (0.80), the application of the Dual-Mode model shows excellent agreement with the sorption data (Figure 7), emphasizing their glassy behavior. In this context, the presence of Langmuir sites is likely attributed to the formation of “microvoids” within the cross-linked polymers synthesized under drastic plasma conditions (PPP > 1). The glassy nature of the polymer is more pronounced for PPP (4.45) than for PPP (0.80), as indicated by the slight deviation in the fit observed at high pressure for PPP (0.80) and the subtle hysteresis loop between the adsorption and desorption branches of toluene for PPP (4.45) [50]. Conversely, the less dense organosilicon films, PPP (0.07) and PPP (0.31), exhibit toluene sorption isotherms that closely follow the Flory-Huggins model (Figure 8). The $\chi$ parameter values obtained for PPP (0.07) and PPP (0.31) (Supporting Information: Table S2) align closely with values reported in the literature for standard polymers [28]. The ability of the Flory-Huggins model to accurately describe the toluene sorption isotherms of PPP (0.07) and PPP (0.31) suggests that plasma polymers prepared under soft plasma conditions (PPP < 1) exhibit sorption behaviors characteristic of conventional elastomers. This rubbery behavior is further demonstrated by the absence of sorption hysteresis during both the increase and decrease of toluene partial pressure, indicating that equilibrium is reached within the time scale of the sorption process [50]. Consistently, polymer swelling upon sorption is reversible and is obviously more pronounced as the density decreases.

From a sensing perspective, the plasma-polymerized organosilicon films deposited on QCMs for toluene detection were evaluated based on key performance indicators. The results demonstrated that PPP (0.07) outperforms the other candidates, exhibiting less pronounced rubbery characteristics.

Typically, sensitivity increases as PPP decreases: the toluene response of PPP (0.07) is more than one order of magnitude higher than that of PPP (4.45). Furthermore, as previously mentioned, the signal of PPP (0.07) is fully reversible upon both increasing and decreasing the partial pressure of toluene. This complete reversibility, which is less pronounced as PPP increases, indicates that the behavior of PPP (0.07) is independent of its state, allowing for precise detection of toluene pressure changes, including both increases and decreases. When the sorbate is replaced by pure water vapor, the devices show no sensitivity to moisture across the entire relative humidity range, consistent with the hydrophobic nature of the films as demonstrated by WCA measurements. Notably, the insensitivity of the devices to water is more pronounced as PPP decreases from 4.45 to 0.07. These findings align with the XPS analysis, which revealed lower O/C ratios as PPP values decreased, indicating a more organic character that is likely to reduce the material's affinity for polar molecules such as water. This trend positions PPP (0.07) as a promising candidate for the selective detection of toluene over water in indoor air conditions, offering a clear advantage over other materials commonly used for VOCs detection. For example, typical Metal-Organic Frameworks face significant challenges due to possible instability when exposed to water [51]. Similarly, zeolites require additional coating processes to achieve the necessary hydrophobicity for selective VOC detection [52]. Having established the sensor's basic performance characteristics, we subsequently focused on the kinetic properties. Response and recovery times were determined from time-resolved curves of PPP (0.07) exposed to abrupt changes in toluene partial pressure (Supporting Information: Figure S12). The QCM exhibits an ultrafast response, with the signal closely tracking changes in toluene partial pressure, as the response and recovery time constants are below the resolution limit of our measurement system (< 1 s). While response times for QCM-based sensors typically range from a few seconds to several minutes, the ultrafast response of PPP (0.07) makes it particularly attractive for real-time monitoring applications.

<figure><img src="image_9.png" /><figcaption>**FIGURE 7** | Adsorption isotherm for toluene in (a) PPP (0.80) and (b) PPP (4.45) at 298 K. The squares are the experimental data and the lines are derived from Equation 6 using the Dual-Mode model, whose parameters are listed in Supporting Information: Table S1.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**FIGURE 8** | Adsorption isotherm for toluene in (a) PPP (0.07) and (b) PPP (0.31) at 298 K. The squares are the experimental data and the lines are derived from Equation 7 using the Flory–Huggins model, whose parameters are listed in Supporting Information: Table S2.</figcaption></figure>

## 4 | Conclusion

This study provides a comprehensive analysis of the structural, chemical, and sorption properties of organosilicon thin films deposited via PECVD under various plasma power parameter (PPP) conditions. Morphological and compositional characterizations using SEM, XPS, and solid-state NMR revealed that the deposition regime significantly influences the polymer structure, transitioning from a soft, rubbery-like behavior at low PPP to a highly cross-linked, glassy nature at high PPP. The increasing mass density with PPP, as measured by XRR, further supports this trend, highlighting the role of plasma energy in material compactness. Additionally, WCA measurements confirmed the hydrophobic character of all films. Crucially, this study establishes a non-trivial and predictive correlation between film structure and behavior for toluene sorption, analyzed using sorption models developed for conventional polymers. Films prepared under soft plasma conditions followed the Flory-Huggins model, indicative of a rubbery behavior, whereas those deposited at higher PPP values were better described by the Dual-Mode model, characteristic of glassy polymers. The most rubbery-like film, PPP (0.07), exhibited the highest toluene sorption capacity, full reversibility, ultrafast signal, and no response to water, making it the most promising candidate for toluene sensing applications. Overall, these findings establish a strong correlation between plasma deposition parameters, film structure, and functional properties, providing valuable insights into the tailored fabrication of plasma-polymerized materials with specific sensing capabilities. Future work could focus on expanding the study to other VOCs to further assess the versatility of these materials for airborne contaminant detection.

### Acknowledgments

We would like to thank Jim Cartier and Christophe Charmette for their technical assistance, Philippe Gaveau for NMR spectra acquisition and Bertrand Rebiere for SEM imaging. XPS and NMR experiments were performed with the support of the Balard Plateforme d'Analyses et de Caractérisation (PAC Balard). We acknowledge the support of LabUM

Chimie at the University of Montpellier and the financial assistance provided by the French government through the National Research Agency (ANR) under the Investments for the Future program (Grant No. ANR-16-IDEX-0006).

### Conflicts of Interest

The authors declare no conflicts of interest.

### Data Availability Statement

The authors have nothing to report.

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## Supporting Information

Additional supporting information can be found online in the Supporting Information section.