Initiated chemical vapor deposition (iCVD) gained an increasing interest over the last years in materials science. In general, a heated stream consisting of monomer- and initiator-species is led over a heated filament plane in a low pressure environment, resulting in the generation of radicals due to thermal decomposition of the initiator. The material to be coated is placed below the filament and is cooled by backside contact with a cooling stage. Upon condensation of the precursors on the surface of the material, radical polymerization takes place. iCVD enables the equipment of sensitive materials with highly functional organic polymers while maintaining the specific topography of the substrate. This makes it possible to create new hybrid materials for a wide range of applications in fields such as organic electronics, biomedical coatings or stimuli-responsive layers

Fig 1. Reaction scheme of iCVD showing thermal decomposition of the initiator followed by radical polymerization directly on the material’s surface. Figure adapted from: H. Moon et al. Nat. Mater. 14, 628 (2015).

Our group focuses on the modification and coating of materials with functional organic thin films for various novel applications. Part of our research consists of modification of surfaces with fluoro-carbon polymers in the context of dielectric applications. Since iCVD represents a technology where no additional solvents or post-reactional steps are needed, highly conformal coatings of PTFE-like polymers are possible. Additionally, even highly porous structures and flexible substrates can be equipped with super-hydrophobic characteristics without losing the specific properties of the substrates.

Fig 2. Schematic representation of hydrophobic iCVD-coatings on porous materials.

Fig 3. Organic thin films made by iCVD on a flexible substrate for use in organic electronics

Deposition of biocompatible hydrogels via iCVD for controlled drug release is part of our research as well. Hydrogels possess the ability to swell upon increase of humidity, or in certain cases by variation of the pH-level, due to the presence of hydrophilic groups inside the polymer. Encapsulation of drugs via wet-chemistry methods represents a challenge in many cases due to the risk of chemical degradation of the drug when interacting with a solvent. This makes iCVD an ideal technique in the field of medical applications. Our work focuses on the coating of water-soluble drugs with cross-linked hydrogels. The variation of the cross-linking degree of the polymer barrier-layer is expected to have fundamental influence on the release kinetics of the coated drugs.

Fig 4. iCVD coating of highly porous ZnO tetrapods with HEMA in the context of controlled drug release.

Recently we were first to demonstrate iCVD of photochromic organic thin films were we incorporated a novel type of chromophore covalently into a polymer matrix. In general, photochromic molecules are able to undergo reversible isomerization between two stable states upon irradiation with light of a certain wavelength. This process results in change of other properties such as dipole moment, molecular size, fluorescence or absorption as well. Exploitation of these properties are highly desirable to be applied to substrates where classical wet-chemistry methods cannot be used. Future undertakings in this context are directed towards mechanochromic thin films acting as optical indicators to mechanical stress.

Fig 5. Photoswitching of the iCVD-produced thin films upon illumination with UV-light where the induced motion of the molecule results in a change of size as well.

Selected publications

S. Schröder, O. Polonskyi, T. Strunskus and F. Faupel. Nanoscale gradient copolymer films via single-step deposition from the vapor phase. Mater. Today, article in press (2020). DOI: 10.1016/j.mattod.2020.02.004

M. H. Burk, S. Schröder, W. Moormann, D. Langbehn, T. Strunskus, S. Rehders, R. Herges and F. Faupel. Fabrication of Diazocine-Based Photochromic Organic Thin Films via Initiated Chemical Vapor Deposition. Macromolecules 53, 1164 (2020).

S. Schröder, T. Strunskus, S. Rehders, K. K. Gleason and F. Faupel. Tunable polytetrafluoroethylene electret films with extraordinary charge stability synthesized by initiated chemical vapor deposition for organic electronics applications. Sci. Rep. 9, 2237 (2019).

Oral C. Aktas, S. Schröder, S. Veziroglu, M. Z. Ghori, A. Haidar, O. Polonskyi, T. Strunskus, K. K. Gleason and F. Faupel. Superhydrophobic 3D Porous PTFE/TiO₂ Hybrid Structures. Adv. Mater. Interfaces 6, 1801967 (2019).

M. Mintken, M. Schweichel, S. Schröder, S. Kaps, J. Carstensen, Y. K. Mishra, T. Strunskus, F. Faupel and R. Adelung. Nanogenerator and piezotronic inspired concepts for energy efficient magnetic field sensors. Nano Energy 56, 420 (2019).