AVS2018 Session PS+PB-MoA: Plasma and Polymers: 'The Legacy of Riccardo d’Agostino and Beyond’

In this talk we present a set of data on the direct PE-CVD of nanometric capsules in the plasma phase resulting from Aerosol Assisted (AA) Atmospheric Pressure (AP) PE-CVD processes fed with Helium (carrier), Ethylene (film precursor) and aerosol droplets of solutions of drugs (nancomicyn, gentamicyn, etc). This approach [1, 2, 3] can lead, in certain conditions, to the deposition of thin coatings directly on the droplets of the aerosol, thus leading to the synthesis of drug-containing nanometric capsules, of potential interest for biomedical applications, e.g., as anti-bacterial surfaces [4].

The influence of various plasma parameters on the AA-AP-PE-CVD processes is discussed, as well as some properties of the coatings.

acknowledgements

The mentorship of Prof. d'Agostino is acknowledged by the authors of this contribution, as well as the recognition of his legacy in the field of plasma processing of materials.

references

1. A. Heyse et al, Plasma Proc. Polym. 8, 965 (2011)

2. Palumbo et al, Plasma Proc. Polym. 12, 1302 (2015)

3. Lo Porto et al, Polym. Chem. 8, 1746 (2017)

4. Sardella et al, Materials 2017, 9, 515

Metal-Organic Covalent Networks (MOCN) were synthesized as ultrathin (<100 nm), defect-free and mechanically flexible layers by initiated Plasma Enhanced Chemical Vapor Deposition (iPECVD). The low substrate temperature used for iCVD allowed use of thermal sensitive porous poly[1-trimethylsilyl)-1-propene] (PTMSP) as a substrate. Uniform layers were grown over 150 nm diameter substrates and did not delaminate upon mechanical deformation. The resulting robust MOCN/PTMSP composite membranes exhibiting outstanding gas separation performance for multiple pairs of gases. Using zinc (II) meso-tetraphenyl porphphyrin (ZnTPP) as the monomer and tert-butyl peroxide as the initiator, film deposition was achieved at low plasma excitation powers. The nearly unchanged intensity of the Soret bands in the UV-vis spectra confirmed the retention of the 18 pi e- conjugation of the monomer units incorporated into the films. The gas selectivity for the separation of H₂/CH₄ or H₂/N₂ is small (<3) for either the PTMSP base membrane or for thin films of evaporated ZnTPP. However, the gas selectivity for both H₂/CH₄ or H₂/N₂ rises to >500 for the PTMSP/iPECVD ZnTPP composite membrane, exceeding the performance of commercial membranes. Ellipsometric Porosimetry and Density Functional Theory Calculations are consistent with a pore size of ~0.4 nm in the MOCN film. Additionally, iPECVD layers derived from a metal-free porphyrin and from porphyrins containing magnanese (III) and cobalt (III) will be described.

ZnO is a wide direct bandgap semiconductor, extensively studied as thin film because of its unique optical, electrical, and piezoelectric properties. Its crystalline structure and degree of crystallinity have a crucial impact on the material characteristics and a particular crystallographic texture may be beneficial for gaining optimized piezoelectric properties or enhanced velocities in surface acoustic wave devices. Therefore, the possibility to control the preferred crystallographic orientation in ZnO thin films has been often investigated.

Atomic layer deposition (ALD) offers the possibility to study in detail the growth of ZnO, due to its layer-by-layer nature and sub-nm thickness control. The initial growth of ZnO adopting diethylzinc (DEZ) and water and its impact on the crystallographic texture of the resulting thin films shed light on the growth mechanisms and the influence of the substrate nature on the in-plane and out-of-plane crystal orientations.

However, thin films deposited with plasma enhanced ALD are known to show several differences when compared to thermal ALD. Therefore, for ZnO thin films the growth mechanism and initial crystallite formation may be greatly influenced by the use of plasma. In this contribution, the initial growth of direct plasma ALD ZnO grown on Si (100) is investigated. DEZ and pure oxygen plasma are adopted during the process. The first 20 cycles of growth are analysed using synchrotron light grazing incidence X-ray diffraction (GIXD), X-ray reflectivity (XRR), and spectroscopic ellipsometry (SE). The influence of substrate temperature ranging from 25 °C to 250 °C on the crystal growth and crystal orientation are studied. A closed layer was obtained within 7 cycles as measured by XRR for all the temperature explored, suggesting island growth of the ZnO. However, GIXD showed no crystalline texture forming at this stage. An amorphous intermixed interface between the native SiO₂ and the growing layer was found, with a density in between pure SiO₂ and ZnO. Irrespective of the deposition temperature, the first crystal structure was measured at around 3 nm, with a preferential out-of-plane orientation transiting from (100) at room temperature to (002) at 250 °C. The first crystallographic seeds were found to largely determine the final microstructure of thicker films.

The utilization of atmospheric pressure dielectric barrier discharges (DBDs) for the surface modification of materials is a very interesting and actual topic. In recent years, efforts have been directed toward the development of a large variety of processes which exploit different DBD configurations and electrode geometries. This contribution describes DBD-based deposition and treatment processes which are potentially of interest also for environmental applications. The processes are performed on polyurethane (PU) foams, i.e., three-dimensional porous substrates which can be quite uniformly surface functionalized by DBDs, since, under suitable experimental conditions, the discharge ignites both outside the foam and throughout its entire porous structure. Three experimental strategies are utilized: i) direct treatment and deposition in parallel plate DBD; ii) direct aerosol-assisted deposition in parallel plate DBD; iii) remote deposition of thin films containing carboxylic acid groups with DBD jet.

The direct treatment is performed by feeding the DBD with He-O₂ mixtures in order to form carboxyl groups on the polyurethane surface. The treated foams are able to adsorb heavy metals from water solutions and can be regenerated (release of adsorbed heavy metals) through immersion in concentrated sodium chloride solution. However, the treated foams suffer ageing in water and, therefore, at the second reuse the amount of adsorbed metals decreases. A water-stable surface containing carboxyl groups is obtained with the DBD jet fed with helium-acrylic acid-ethylene mixtures (remote thin film deposition). No morphological and chemical modifications of the deposits are detected also after 72 h of immersion in water.

When the parallel plate DBD is fed with He-C₃F₆ mixtures, the deposition of fluoropolymer thin films occurs on both the exterior and interior of the foam. The treated foams are superhydrophobic and oleophilic, simultaneously, with potential application for the separation of oil from polluted water, since they selectively adsorb oil, while repelling water.

The direct aerosol-assisted deposition in parallel plate DBD is utilized to prepare hybrid organic-inorganic nanocomposite coatings composed of a hydrocarbon polymer and ZnO or TiO₂ nanoparticles. Under appropriate deposition conditions, it is possible to obtain multifunctional coating characterized by superhydrophobicity, superoleophilicity as well as by the peculiar photocatalytic properties of the utilized nanoparticles. The application of the treated foams for oil-water separation and for the photocatalytic degradation of organic pollutants in water is currently under investigation.