New Electronic and Multifunctional Polymer Thin Films Enabled by Initiated Chemical Vapor Deposition
The aim of this work was the establishment of initiated chemical vapor deposition (iCVD) at the Chair for Multicomponent Materials (Prof. Dr. Franz Faupel) and the development of iCVD thin film electrets for biomagnetic sensors. The iCVD process developed by Gleason et al. enables a high-precision film growth control and control of the resulting film functionality of the polymer thin films due to the CVD-typical growth characteristics and the solvent-free radical polymerization from the vapor phase. Based on the work of Gleason et al., the iCVD process is newly established and further developed as the first objective of this work at the Chair for Multicomponent Materials. In order to obtain a more detailed understanding of the underlying reaction processes and to improve the process control, an in-situ mass spectrometry extension for the iCVD process, which is newly developed in the course of this work, is also presented. Starting from simple insulators, which can currently be deposited by iCVD, the next objective is to investigate whether it is possible to produce so-called thin film electrets by iCVD. Electrets are functional dielectrics that can store a charge over a very long period of time and thus provide a (quasi)permanent electric field over a long period of time, much like a permanent magnet provides a magnetic field for a long period of time. Their versatile field of application ranges from electret microphones to energy generators and electrostatic air filters. Within the scope of this work, the electrets are intended for new electrostatic magnetic field sensors, which are developed in close cooperation with the Chair for Functional Nanomaterials (Prof. Dr. Rainer Adelung) as project A2 within the Collaborative Research Center (CRC) 1261. For this purpose, the long-standing experience in the field of thermal evaporation of Teflon AF thin film electrets at the Chair for Multicomponent is used and, among other things, it is investigated whether the iCVD fluoropolymers enable a further improvement of the charge carrier stability as well as better film control by the CVD-typical growth conditions. The subsequent objective is dedicated to the question of how iCVD electrets can be further developed and tailored for the application in sensors. This can only be achieved by a better understanding of the underlying charge storage mechanisms. Therefore, the influence of different end groups on the charge storage properties, enabled by the individual tunability of the film functionality in the iCVD process, is investigated first. In addition, new organic iCVD electret multilayers are demonstrated to specifically address challenges that may arise in connection with the electret component in sensors. Furthermore, an approach is demonstrated that allows the formation of microporous polymer films by phase separation during deposition, which can increase the effective surface charge. Finally, to complete the field, it is investigated whether it is possible to deposit conjugated thin films by iCVD via new acetylene-like monomers in contrast to the typical insulating iCVD films. Adhesion problems that occur during deposition, especially with fluoropolymer films, are finally solved by novel gradient copolymer films inspired by nature. The chemical composition of these films changes from polymer type A to polymer type B along the film thickness. With the help of the newly developed in-situ mass spectrometry extension, the deposition of the new nanoscale gradient copolymer films with film thicknesses below 30 nm is finally made possible. A combination of two materials in one material represents a completely new type of material in terms of physical and chemical properties. It not only enables improved adhesion, but can also pave new paths for organic electronics, future sub-wavelength devices and the replication of natural gradient structures, for example for molecular machines on the lower nanoscale.
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