Electrochromicity and Slanted Columnar Thin Films: A Combinational Studies Approach

Slanted columnar thin films (SCTFs) are high surface area nanostructured thin films fabricated by glancing angle deposition, a physical vapor deposition technique. The properties of SCTFs have given rise to their use as substrates for optical, biological, catalysis, and many other research avenues. The highly ordered, spatially coherent arrangement of the constituent nanostructures which comprise the SCTF exhibit strong anisotropic optical properties. These anisotropic optical properties are observable with generalized spectroscopic ellipsometry (GSE), an optical analysis technique capable of determining SCTF optical and structural parameters through a best-match model approach. Recent investigations have shown that SCTFs are highly applicable for use in liquid and flow-cell experiments. Accordingly, electrochemically (EC) induced changes to the composition of the SCTFs or to functionalized coatings conformally coating the SCTFs are capable of producing optically anisotropic electrochromic responses, observable with GSE and verifiable with complementary analysis techniques. In this work, the growth and preparation of SCTFs passivated by ultra-thin layers, either by atomic layer deposition or functionalized self-assembled monolayers, are presented. Analysis techniques for optical and structural characterization of the SCTFs are detailed. The design and experimental application protocols for the in-situ EC-GSE liquid cell and the in-situ tribrid (EC-GSE-quartz crystal microbalance with dissipation) flow cell are provided. Lithium-ion intercalation and deintercalation of silicon-alumina core-shell SCTFs are stimulated by cyclic voltammetry and monitored with GSE. The compositional changes of the SCTF electrode produce strong electrochromic changes, which are observed in the anisotropic optical constants of the SCTF when elicited with a best-match model incorporating the homogeneous biaxial layer approach. The analysis results provide insight into the morphological changes, specifically swelling and metalization, taking place throughout the lithium-ion intercalation and deintercalation processes. In addition, electrochemically functionalized self-assembled monolayers are deposited onto gold SCTFs and subsequently subjected to a series of electrochemical chronoamperometry steps while under observation of GSE and quartz crystal microbalance with dissipation. Optical properties of the electrochromic redox reaction are presented. A combinational analysis is undertaken incorporating a parameterized best-match GSE model and complementary finite element method simulations to discern the effects of the SCTF geometry on the functionalized self-assembled monolayer electroactivity