Plasma Polymer Films Research Articles

Swellable Plasma Polymer Films for Use in Hydrogel-Based Biomedical Devices

Swellable Plasma Polymer Films for Use in Hydrogel-Based Biomedical Devices

Investigating the Influence of the Substrate Temperature and the Organic Precursor on the Mechanical Properties of Low‐Pressure Plasma Polymer Films

ABSTRACTThis work aims to investigate the influence of the substrate temperature on low‐pressure plasma polymerization processes. Emphasis is placed on the mechanical properties and growth mechanism of the plasma polymer films (PPFs). For this purpose, two precursors are considered, differing only by their unsaturation degree: 2‐propen‐1‐ol (CH2═CH–CH2–OH) and propan‐1‐ol (CH3–CH2–CH2–OH). Although propan‐1‐ol‐based PPFs behave like hard elastic solids, 2‐propen‐1‐ol‐based coatings evolve from a liquid film to an elastic solid on increasing the substrate temperature. This behavior is understood considering the evolution of the glass transition temperature of PPFs. The latter is correlated with the cross‐linking degree of the polymeric network governed by the energy density of bombarding ions on the growing film.

Characterization of plasma polymerized acetonitrile film for fluorescence enhancement and its application to aptamer-based sandwich assay.

Among biosensing systems for sensitive diagnoses fluorescence enhancement techniques have attracted considerable attention. This study constructed a simple multilayered structure comprising a plane metal mirror coated with a plasma-polymerized film (PPF) as an optical interference layer on a glass slide for fluorescence enhancement. Plasma polymerization enables the easy deposition of organic thin films containing functional groups, such as amino groups. This study prepared PPFs using acetonitrile as a monomer, and the influences of washing and the output powers of plasma polymerization on PPF thickness were examined by Fourier transform infrared spectroscopy. This is because controlling the PPF thickness is vital in fluorescence enhancement. Multilayered glass slides prepared using a silver layer with 84 nm-thick acetonitrile PPFs exhibited 11- and 281-fold fluorescence enhancements compared with those obtained from the substrates with a bare surface and only modified by the silver layer, respectively. Oligonucleotides labeled with a thiol group and cyanine5 were successfully immobilized on the multilayered substrates, and the fluorescence of the acetonitrile PPFs was superior to that of the allylamine and cyclopropylamine PPFs. Furthermore, an aptamer-based sandwich assay targeting thrombin was performed on the multilayered glass slides, resulting in an approximately 5.1-fold fluorescence enhancement compared with that obtained from the substrate with a bare surface. Calibration curves revealed the relationship between fluorescence intensity and thrombin concentration of 10-1000 nM. This study demonstrates that PPFs can function as materials for fluorescence enhancement, immobilization for biomaterials, and aptamer-based sandwich assays.

Unlocking the Potential of Liquid Plasma Polymer Films: Characterizing Aging Effects and Their Impact on the Wrinkling Phenomenon.

Here, we present the study of the intricate dynamics between the physicochemical properties of liquid propanethiol plasma polymer films (PPFs) and the formation of wrinkles in PPF/Al bilayers. The study investigates the effect of liquid PPF aging in the air before top Al layer deposition by magnetron sputtering on the wrinkling phenomenon for 4 days. Thanks to atomic force microscopy, the wrinkle dimensions were found to decrease by approximately 55% in amplitude and 66% in wavelength, correlated with an increase in the viscosity of the PPF over the aging duration (i.e., from less than 107 to 1010 Pa·s). This behavior is not linked to alterations in cross-linking degree as evidenced by time-of-flight secondary ion mass spectrometry experiments but rather to network densification driven by the inherent molecular chain mobility due to the viscous state of the PPF. X-ray photoelectron spectroscopy measurements emphasizing the absence of oxidation of the PPF over the aging duration support this, revealing a unique aging mechanism distinct from other plasma polymer families. Overall, this study offers valuable insights into the design and application of mechanically responsive PPFs involved in bilayer systems, paving the way for advancements in nanotechnology and related fields.

Optical characterization and dispersion analyses of plasma polymerized methyl acrylate thin films

This work reports the structural characteristics, surface morphology, linear and nonlinear optical properties of 110 to 225 nm thick plasma polymerized methyl acrylate (PPMA) thin films. X-ray diffraction analyses confirm the amorphous nature of the films. Field emission scanning electron micrographs of the films display cluster-based surface morphology. Attenuated total reflectance Fourier transform infrared spectroscopy confirms the chemical structural changes in the films. The optical properties were studied based on the absorbance, transmittance, and reflectance spectra measured by an ultraviolet–visible spectrophotometer within the wavelength ranges from 200 to 800 nm. The direct optical band gap and Urbach values are increased from 3.66 to 3.83 eV and 0.28 to 0.45 eV, respectively with increasing film thickness. The extinction coefficient and refractive index were evaluated, and discussed a correlation between the refractive index and the optical bandgap. The real and imaginary dielectric constants, volume/surface energy loss functions and skin depth were deduced. The oscillator energies and parameters were analyzed using the concept of Wemple-DiDomenico and Sellmeier models, respectively for a single oscillator. Static linear refractive index for the studied films exhibits normal dispersion behavior with film thicknesses and satisfied Moss, Ravindra-Gupta, and Herve-Vandamme rules. The linear susceptibility, third-order nonlinear susceptibility and the non-linear refractive index are considerably reduced from 0.20, 29.5 × 10−14 esu, and 5.89 × 10−12 esu with increasing optical band gap energies. The outcomes from the analyses of PPMA demonstrated their potential for usage in electronic, optoelectronic, and non-linear device applications.

A combinatorial hollow cathode discharge/plasma polymerization system for silver containing plasma parylene coating

The conventional pyrolysis of parylene coating depends on a high vacuum environment and an expensive solid dimer precursor. Our previous study successfully demonstrated the feasibility of synthesizing plasma parylene (PP) coating using plasma polymerization with a liquid para-xylene precursor. The deposition conditions for plasma polymerization are compatible with the vacuum level of the hollow cathode discharge (HCD). This led to the idea of using an HCD gun as the metal source to incorporate metal into the plasma polymerized films, thereby achieving different coating functions and extending applications, such as active antibacterial purposes. In this study, an HCD gun was installed on the plasma polymerization reactor to synthesize a silver containing plasma parylene (Ag-PP) coating, where the silver HCD tube serves as the in situ source for incorporating silver.The experimental results demonstrate that the quantity of incorporated silver increases linearly with the power delivery level of the HCD device, and its content can be controlled during deposition to form an Ag-PP coating. Microstructural analysis reveals that the incorporated silver exhibits a crystalline and granular structure with a metallic state in the Ag-PP coating. Furthermore, the Ag-PP coating significantly resists both Staphylococcus aureus and Escherichia coli, with an antibacterial activity value exceeding 2.0 after 24 h of plate culture, corresponding to a silver metal ion release of 2–3 ppb. This quantitatively substantiates that the incorporated silver can impart an active antibacterial function to the PP coating for biomedical surface treatment applications.

Comparison of Plasma-Polymerized Thin Films Deposited from 2-Methyl-2-oxazoline and 2-Ethyl-2-oxazoline: I Film Properties.

Poly(2-oxazoline) is a promising new class of polymeric materials due to their antibiofouling properties and good biocompatibility. Poly(2-oxazoline) coatings can be deposited on different substrates via plasma polymerization, which can be more advantageous than other coating methods. The aim of this study is to deposit poly(2-oxazoline) coatings using a surface dielectric barrier discharge burning in nitrogen at atmospheric pressure using 2-methyl-2-oxazoline and 2-ethyl-2-oxazoline vapours as monomers and compare the film properties. For the comparison, the antibacterial and cytocompatibility tests were peformed according to ISO norms. The antibacterial tests showed that all the deposited films were highly active against Staphylococcus aureus and Escherichia coli bacteria. The chemical composition of the films was studied using FTIR and XPS, and the film surface's properties were studied using AFM and surface energy measurement. The cytocompatibility tests showed good cytocompatibility of all the deposited films. However, the films deposited from 2-methyl-2-oxazoline exhibit better cytocompatibility. This difference can be explained by the different chemical compositions and surface morphologies of the films deposited from different monomers.

Polymerization mechanisms of hexamethyldisiloxane in low-pressure plasmas involving complex geometries

Hexamethyldisiloxane (HMDSO) low-pressure plasmas are known for their versatility in the deposition of plasma polymer films (PPFs) with different properties and applications. Although they have been studied for decades, the reaction mechanisms of plasma polymer formation leave open questions, particularly when deposition on 3D materials with complex geometries such as cavities and undercuts is considered. In the present study, two configurations named “cavity” and “undercut” have been selected to study the influence of diffusion of film-forming species and surface reactivity in HMDSO plasmas without and with O2 admixture. A varying spatial chemical composition of the plasma polymer deposit along the penetration depth of the studied configurations indicates different sticking probabilities of the film-forming species. Furthermore, although ion-induced effects are usually only considered for direct plasma exposure, the obtained results and additional etching experiments reveal that the contribution of high-energy particles might still be considered underneath small openings. Finally, the relevance of oxidizing chemical reactions at the surface inside the configurations is clarified when O2 is added to the plasma.

Charge carrier transport mechanism in plasma polymerized methyl acrylate thin films

Plasma-polymerized methyl acrylate thin films (PPMA) of thicknesses 115 to 290 nm were prepared using a capacitively coupled glow discharge plasma reactor. The current density-voltage (J-V) characteristics of the Aluminum/PPMA/Aluminum sandwich configuration were studied between 0.1 and 75 V over the temperature region from 298 K to 398 K for various thicknesses of PPMA thin films. J-V curves show that the conduction current follows Ohm's law in the low voltage area, but that in the high voltage range, the behavior is attributed to be space charge-limited conduction (SCLC) in PPMA thin films. The free carrier density, the trap density and carrier mobility have been determined to be around 1.77 × 1023 to 5.86 × 1023 m−3, 2.84 × 1024 to 3.77 × 1024 m−3, and 3.87 × 10−18 to 69.7 × 10−18 m2V−1s−1, respectively for various thicknesses of PPMA thin films. For the higher temperature and lower temperature areas, the activation energies were found in the range from 0.60 to 0.84 eV and 0.28 to 0.40 eV, respectively for 5 V in Ohmic region and from 0.61 to 0.98 eV and 0.27 to 0.34 eV, respectively for 60 V in SCLC (Non-Ohmic) region, respectively. It is observed that SCLC mechanism in PPMA thin films is dominated by a single dominant trap level/discrete trap level distribution. The PPMA thin films are promising candidates in manufacturing optoelectronic devices.

Designing Nanostructured Organic-Based Material by Combining Plasma Polymerization and the Wrinkling Approach.

In this work, an innovative and versatile strategy for the fabrication of nanostructured organic thin films is established based on the wrinkling phenomenon taking place in a bilayer system constituted by a liquid plasma polymer film (PPF) and a top Al coating. By means of morphological characterization (i.e., atomic force microscopy and scanning electron microscopy), it has been demonstrated that the wrinkle dimensions (i.e., wavelength and amplitude) evolve as a function of the PPF thickness according to models established for conventional polymers. The wrinkled surfaces exhibit great stability over time as their dimension did not vary after 100 days of aging, resulting from a pinning phenomenon between the Al layer and the Si substrate, hence freezing the morphology. In a second step, the wrinkled surfaces have been employed as templates for the deposition of an additional PPF third layer, giving rise to the formation of a nanostructured organic-based surface. The chemical composition of the material can be tuned through an appropriate choice of precursor (i.e., allyl alcohol or propanethiol).

Effects of C4F8 plasma polymerization film on etching profiles in the Bosch process

The Bosch process is a deep etching method for silicon that uses C4F8 plasma-deposited polymerized films as passivation films to protect the silicon sidewalls. This study measured the deposition rate of the passivation films and the etch rate with F-radical exposure and analyzed the chemical composition of the films. Additionally, we observed the deformation of the passivation films during the Bosch process and assessed its influence on the etch profiles. As the C4F8 flow rates increased, the deposition rates attained a local maximum, subsequently decreased to a local minimum and then increased again. The deposition rates were extremely low when the pressure exceeded 10 Pa. With the increasing C4F8 flow rates, inductively coupled plasma power, and pressure, the respective bond content varied up to 10%, and C—CFX and C—C bond contents were replaced with CF2 and CF contents, respectively. The results indicated that the chemical composition of the films did not affect the etch rates of the films, and upon exposure to F radicals, the chemical composition of all films transformed into an identical chemical composition with a higher CF2 bond content. Polymerized films with low CF2-bond content deformed with F-radical exposure, enabled the passage of F radicals, and did not serve as passivation films. In addition to high deposition rates and high F-radical resistance, the Bosch process requires passivation films with high CF2 bond content. The present findings will aid in tuning the parameters of the Bosch process and increase the productivity of silicon deep reactive-ion etching.

The influence of the substrate temperature on the growth mechanism of amine‐ and thiol‐based plasma polymers: A comparative study

AbstractThis work aims to provide new insights into the link between the growth mechanisms of functionalized plasma polymer films (PPFs) and the substrate temperature (TS). By means of AFM‐based techniques, it has been demonstrated that the mechanical behavior of the coatings is dramatically affected by TS and the precursor employed (i.e., 1‐propanethiol or 1‐propylamine). While propylamine‐based PPFs behave as hard elastic materials regardless of TS, propanethiol‐based PPFs evolve from viscous liquids to elastic solids with increasing TS. This behavior can be understood considering the glass transition temperature of PPF. For both precursors, the latter is correlated to the cross‐linking density controlled through the energy density brought by positive ions to the growing film.

Enhanced ammonia/amines sensitivity at room temperature using plasma polymerized polyvinyl acetate-reduced graphene oxide composite film sensors

Amines and ammonia can harm the environment, even in small concentrations. Efficient amine sensors are required to monitor the quality of food and air. A growing number of industries need sensors that can recognize a particular gas. Here, we demonstrate composite films made of plasma polymerized vinyl acetate (PVAc) and reduced graphene oxide (rGO) to address the crucial issues of limited selectivity, long-term instability, and temperature instability in chemoresistive gas sensors. Change in resistance of the sensor was studied in presence of analyte gas. An inductively linked plasma polymerization setup prepares PVAc films under carefully calibrated deposition conditions. Several samples are analysed to determine the ideal PVAc to rGO ratio for the composite film. Compared to individual PVAc and rGO films, final samples have demonstrated a sensitivity to ammonia and amines up to 15 times higher. The sensitivity of measuring ammonia/amines ranges from 628 for trimethylamine to a maximum of 3389 for methylamine. The sensitivity of measuring ammonia is 1621, while the sensitivity of measuring dimethylamine is 2041. The redox reaction, charge transfer, and swelling of polymer films explain potential sensing processes. Ammonia/amine selectivity has been enhanced by up to 160 compared to other volatile organic compounds (VOCs) using PVAc-rGO composite films. Based on unique selectivity, these films may also successfully distinguish between ammonia and other amines. It is shown that fluctuations in temperature between 30 °C and 150 °C did not affect the sensitivity of PVAc-rGO films. These films remain sensitive for far longer than three months. As a result, composite films of PVAc-rGO may be used to fabricate exceptionally sensitive and reliable gas sensors for ammonia/amines environmental monitoring.

Complementary spectroscopic and electrochemical analysis of the sealing of micropores in hexamethyldisilazane plasma polymer films by Al2O3 atomic layer deposition

In the present study, the effects of oxygen plasma treatment and subsequent 2 nm thin Al2O3 film deposition by atomic layer deposition on about 30 nm thick hexamethyldisilazane polymer layers are investigated by using a combination of spectroscopic and electrochemical analysis. The investigations focus on the microporosity of the corresponding films and their structural changes. Upon oxygen plasma treatment, the surface near region of the films is converted into SiOx, and the microporosity is increased. Atomic layer deposition of Al2O3 on the plasma oxidized films leads to the decrease of pore sizes and an effective sealing. A correlation between the film microporosity and the change of hydroxyl groups of the films with the adsorption of water was established by ellipsometric porosimetry and in situ Fourier transform infrared (FTIR) spectroscopy. Moreover, electrochemical analysis provided complementary information on the electrolyte up‐take in the differently conditioned thin films.

Plasma Polymerized Organosilicon Thin Films for Volatile Organic Compound (VOC) Detection

Plasma polymerized (PP) thin films deposited in a soft or intermediate plasma discharge from hexamethyldisiloxane (HMDSO) were developed as sensors for the detection of volatile organic compound (VOC) vapors. Energy dispersive X-ray spectroscopy (EDX) and X-ray reflectometry (XRR) were performed to determine the organosilicon films’ elemental composition and density. Spectroscopic ellipsometry measurements were carried out to determine the refractive index of the films. Quartz crystal microbalance (QCM) and ellipsometry coupled to vapor sorption were used to investigate the sorption mechanism of several VOC vapors into the films as a function of the plasma deposition conditions. The density and the refractive index of the PP-HMDSO films increased with the plasma energy due to a different chemical composition and different proportion of free volumes in the material network. The PP-HMDSO films showed different affinities towards the VOC vapors depending on the plasma discharge energy. The films elaborated in the lowest plasma energy revealed a good sensitivity towards the VOCs, especially toluene (one of the BTEX vapors), compared to the other films deposited under higher plasma energy. In addition, the selectivity between toluene and other non-BTEX VOCs such as heptane and ethanol decreased to become zero while increasing the plasma energy.

Impact of plasma polymerized Iodine-Doped thiophene films for enhanced sensing response towards industrial VOCs

Volatile organic compounds (VOCs) are majorly used in the manufacturing industry and are hazardous to the sustainability of the environment. Polymeric sensors, rather than more traditional metal oxide sensors, are currently taking the lead attention in the greater part of VOC sensing investigations. In polymeric sensors, plasma polymerized thiophene (PPTh) films are being investigated because of their unique properties, wasteless fabrication, and feasibility for commercial mass manufacturing. In the present study, we fabricated PPTh films powered by a 13.6 MHz RF generator with a vacuum level of 10-3 Torr in a home-built plasma system. The gas sensor investigation of PPTh and iodine-doped PPTh samples with various VOCs, such as ethanol, methanol, acetone, benzene, toluene, and dichloromethane, was carried out systematically. The iodine-doped PPTh 10–20 films (48 hr) showed the highest sensitivity for ethanol (∼180) and methanol (∼120) as compared to PPTh for ethanol (∼110) at 1000 ppm gas concentration. A sensitivity degradation study over 60 days revealed the long-term stability of the films in the air. The polar hopping mechanism and decreased charge transport centers after reaction with the hydroxyl group are responsible for resulted sensitivity of iodine-doped PPTh films towards alcohols.

Change in Electrical/Mechanical Properties of Plasma Polymerized Low Dielectric Constant Films after Etching in CF4/O2 Plasma for Semiconductor Multilevel Interconnects.

As semiconductor chips have been integrated to enhance their performance, a low-dielectric-constant material, SiCOH, with a relative dielectric constant k ≤ 3.5 has been widely used as an intermetal dielectric (IMD) material in multilevel interconnects to reduce the resistance-capacitance delay. Plasma-polymerized tetrakis(trimethylsilyoxy)silane (ppTTMSS) films were created using capacitively coupled plasma-enhanced chemical vapor deposition with deposition plasma powers ranging from 20 to 60 W and then etched in CF4/O2 plasma using reactive ion etching. No significant changes were observed in the Fourier-transform infrared spectroscopy (FTIR) spectra of the ppTTMSS films after etching. The refractive index and dielectric constant were also maintained. As the deposition plasma power increased, the hardness and elastic modulus increased with increasing ppTTMSS film density. The X-ray photoelectron spectroscopy (XPS) spectra analysis showed that the oxygen concentration increased but the carbon concentration decreased after etching owing to the reaction between the plasma and film surface. With an increase in the deposition plasma power, the hardness and elastic modulus increased from 1.06 to 8.56 GPa and from 6.16 to 52.45 GPa. This result satisfies the hardness and elastic modulus exceeding 0.7 and 5.0 GPa, which are required for the chemical-mechanical polishing process in semiconductor multilevel interconnects. Furthermore, all leakage-current densities of the as-deposited and etched ppTTMSS films were measured below 10-6 A/cm2 at 1 MV/cm, which is generally acceptable for IMD materials.

Atmospheric Pressure Plasma Polymerization of Carvone: A Promising Approach for Antimicrobial Coatings

Medical devices are often vulnerable to colonization by nosocomial pathogens (bacteria), leading to infections. Traditional sterilization methods may not always be effective, and as a result, alternative options are being explored to prevent microbial contamination. Recently, scientists are emphasizing using plant-derived essential oils that possess inherent antibacterial properties to produce antimicrobial coatings using plasma polymerization technology carried out at atmospheric pressure (AP). This approach shows promise compared to other coating strategies that need several processing steps, including a high-vacuum system, and are laborious, such as the immobilization of antimicrobial materials on precoated layers in the low-pressure plasma polymerization approach. The present study demonstrates the potential of AP plasma polymerization for producing thin films with excellent antibacterial properties and surface characteristics. The resulting coatings are stable, smooth, and have high wettability, making them ideal for repelling bacteria. The calculated zeta potential and deposition rate for the films are also favorable. These AP plasma-polymerized thin films created from carvone show a reduction rate of more than 90% for Escherichia coli and Staphylococcus aureus bacteria. Our computational docking studies also reveal strong binding interactions between the original carvone monomer and both bacteria. The study suggests that these AP plasma-produced coatings have great potential as antibacterial coatings for biomedical devices.

Deposition and morphology of direct current plasma-polymerized aniline

Plasma polymerization is a simple, solvent-free, dry process that involves vaporizing a monomer and ionizing them to the plasma state. This study reports the plasma polymerization of aniline in a direct current glow discharge. Aniline was introduced into a vacuum chamber and plasma-polymerized onto an indium-doped tin oxide/glass substrate at varying discharge potentials and deposition times. The resulting plasma-polymerized aniline films were in the leucoemeraldine form, as evidenced by UV-Vis and infrared spectral analyses. Monocarbon species that are products of the aromatic ring rupture were observed from the optical emission spectra of the discharge. The films reveal a smooth, pinhole-free surface across different process parameters. The film thickness from the 200 to 600 nm range presents a linear relationship with respect to the duration of deposition. The deposition rate also increased and has a good linear relationship when the discharge potential was varied.

Plasma-Polymerized and Iodine-Doped Polyvinyl Acetate for Volatile Organic Compound Gas Sensing Applications

Nowadays, the increased emission of hazardous volatile organic compounds (VOCs) due to rapid growth in the manufacturing industry has enforced demand for highly sensitive, selective, and stable gas sensors. Among the different types of gas sensors, we resolved issues associated with chemoresistive alcohol sensors like low selectivity, temperature instability, and long-term instability using iodine-doped polyvinyl acetate (IPVAc) films. These polymer-based chemoresistive sensors will be beneficial for low power consumption, ease of manufacture, and long-term stability owing to their simple structure and operation. The polymer films were prepared using the inductively coupled tubular plasma polymerization method. These films were doped with iodine at room temperature. The physicochemical characterization of prepared films confirmed the uniform coating of granular type morphology, iodine doping, and cubical crystal structure. The obtained maximum sensitivity was 122 and 7360 a.u. for 1000 ppm of methanol gas for polyvinyl acetate (PVAc) and IPVAc films, respectively. Also, IPVAc films demonstrated sensitivities of 5554 and 1995 a.u. for 1000 ppm of each ethanol and 2-propanol gas, respectively. IPVAc films illustrated the highest selectivity of 600 a.u. for alcohols over other VOCs like acetone, benzene, toluene, and dichloromethane. The high electronegativity of IPVAc films due to iodine doping is the key origin of the highly selective adsorption of VOCs. Additionally, the IPVAc films have stable sensitivity performance over the temperature range of 30–130 °C and repeatability over 60 days. Thus, the plasma-polymerized IPVAc films can be applied for fabricating highly selective, sensitive, and stable VOC sensing applications.