ICMCTF 2025 Session CM1-1-ThM: Spatially-resolved and in situ Characterization of Thin Films, Coating and Engineered Surfaces I

In this presentation, we will present the results of deuterium distribution and quantitative analysis by atom probe tomography (APT) in single-layered V or Fe/V multi-layered films. While V is a hydrogen-absorbing metal, Fe hardly dissolves hydrogen (H). Therefore, in Fe/V multi-layered films, almost all of the H atoms are supposed to be in the V layer, and the H distribution should show a clear contrast at the Fe/V interface. This is also the case for deuterium (D). D has a lower diffusion coefficient than H. Hence, a surface segregation of D during the APT analysis can be suppressed, enabling to visualize original D position in the host metal lattice. Furthermore, D can be distinguished from background hydrogen in the APT analysis chamber. By using a portable chamber to prevent the sample from being exposed to oxygen, we were able to measure the D concentration in V or Fe/V films over a wide concentration range. We compared the D concentration measured by APT with the compared with the results of measurements of the hydrogen concentration dependence of electromotive force (EMF) for similar samples, it was found that the average D concentration in the sample could be correctly evaluated using APT.

The electrochemical energy storage behavior of nano- and microscale (thin- and thick-film) electrodes displays unique characteristics that provide crucial insights into various charge storage mechanisms, essential for the optimal design of commercial battery applications. Additionally, these films are applicable for constructing microbatteries for miniature electronic devices (such as sensors). Critically, material chemistry, crystallinity, and nanostructure significantly influence active charge transfer mechanisms in these systems, generally classified as electrochemical double layer capacitive, pseudocapacitive, or battery-type behaviors. In lithium-ion batteries specifically, the charge storage mechanism involves the (de)/intercalation of lithium ions in active electrode materials, such as silicon, graphite, or transition-metal oxides.

By optically monitoring thin-film electrodes under electrochemical charge/discharge, a range of time-resolved structural data can be obtained. This work elaborates on this by integrating operando spectroscopic ellipsometric data acquisition. Ellipsometry, highly sensitive to thin films, offers an advantage by effectively excluding any electrolyte side-reactions from measurement, thus providing accurate, real-time data on the evolving structure of lithiated electrodes across charging states. Additionally, ellipsometry tracks thickness changes, enabling precise monitoring of degradation mechanisms.

In this study, thin film samples of Pd-Ti and Ni-Ti systems were prepared using an electron beam evaporator under a vacuum pressure of 10^-6 torr on Si<100>, borosilicate glass, and pure Ti substrates. The absorption of hydrogen, carbon, and oxygen into the thin films was followed by conducting an in-situ real-time Rutherford backscattering (RBS) investigation. RBS confirmed no spontaneous interdiffusion of atoms between the layers during the deposition. The layers were approximately ~30×10¹⁵ atoms/cm² (10 nm), and ~433×10¹⁵ atoms/cm² (150 nm) for Pd, and Ti layers in the Pd-Ti system, respectively while Ni layer was ~31×10¹⁵ atoms/cm² (10.3 nm). The in-situ real-time RBS was performed by linear temperature ramping from room temperature to a maximum temperature of 600 ^oC at a constant ramping rate of 7 ^oC per minute. The results showed an interfacial reaction, indicating an unstable system at higher temperatures. Elastic recoil detection analysis (ERDA) revealed an increase in hydrogen absorption of up to 5 at% from the residual gases inside the scattering chamber at a vacuum of 1×10^-6 mbar. Increased oxygen peaks indicated enhanced absorption of this element from the residual gases inside the scattering chamber. These results suggest that this system has potential applications in hydrogen absorption.