Exploring the envelope of physical vapor deposition: Nano- and microstructured films for electrochemical applications

Physical vapor deposition (PVD) methods are established scalable industrial processes for creating high-quality functional films in various industries. PVD deposited films are commonly dense and smooth with structural features in the nanoscale, but by variation of deposition parameters, film properties and structure can be modified to enhance application-specific performance. In electrode materials for electrochemistry, for instance, a larger accessible surface area commonly increases the number of electrochemical reactions occurring at the same time on the electrode. In this work, PVD methods are used to deposit film materials for electrochemical applications: Nanostructured carbon thin films are investigated as electrochemical biosensors, and microstructured titanium oxide films are demonstrated in microbattery and photocatalysis applications. The aim is to explore and determine how PVD methods can be utilized to construct film structures with a high degree of application-specific tailorability in terms of nano- and microstructural features. Comprehensive structural and physicochemical characterization is carried out for the deposited materials, providing the fundamental tools and insight required to understand and link the observed application performance to changes in material properties. In the first part, the nanostructure of thin and ultrasmooth chemically inert carbon films high in sp3-bonded carbon are modified by alloying with iron, doping with nitrogen, and by embedding carbon nanodiamonds into the film structure. The addition of iron into the films as well as doping with nitrogen are found to enhance the performance of electron transfer on the carbon electrodes in electrochemical sensing applications. It is however also found that these modifications open and alter the initially high sp3-carbon nanostructure, exposing it to atmospheric contaminants. In the second part, a higher gas pressure is applied during PVD deposition, inducing cluster and nanoparticle formation of the deposited material. This gas nucleation technique is leveraged with titanium oxide as the deposition material to construct thick and porous microstructures. In the first subsection, a large permanent magnet is used to collect and self-assemble the gas nucleated titanium oxide nanoparticles into a hierarchical film structure several micrometers in thickness comprising particle clusters of varying sizes. This microstructure offers a considerably large specific surface area, which is important in its application as a photocatalyst. In the second subsection, in an otherwise conventional PVD process, substrate biasing in combination with gas nucleation of lithium-titanate-carbon material is found to result in an unidentified growth mechanism of micrometer-sized pillars. The performance of this micropillar structure is demonstrated as an anode material in Li-ion microbattery. These findings showcase the adaptability of conventional PVD methods for depositing films with diverse nano- and microscale features. Thorough characterization is essential for understanding material changes and in uncovering unidentified phenomena in films deposited via physical vapor deposition.