Jacobus M. Sturm

We characterize the shape of the surface peaks and subsurface signal in low energy ion scattering (LEIS) spectra of calcium niobate (Ca2Nb3O10) nanosheets and sub-nanometer amorphous Nb-oxide films. By analyzing LEIS spectra acquired at different primary ion energies, we show that channeling effects significantly contribute to the spectral shape of the nanosheet sample. In addition, by depositing Si films of varying thicknesses on the nanosheets, we track the evolution of the Nb signal as a function of depth. Together, these results demonstrate how the crystalline structure of the sample influences the LEIS response. Since common LEIS simulation models do not account for the crystal structure, our findings highlight the importance of understanding the sample’s microstructure prior to simulating LEIS spectra.

Hydrogen, crucial for the green energy transition, poses a challenge due to its tendency to degrade surrounding wall materials. To harness hydrogen's potential, it is essential to identify the parameter(s) of materials that modulates hydrogen-material interaction. In a recent publication, we have shown that the reduction (denitridation) of transition metal (TM) nitrides in hydrogen radicals (H*) stops when their work function drops below a threshold limit. In this work, we tailor the work function of a complex TM oxide by tuning the relative contents of its constituent TM atoms. We show that increasing the fraction of a low-work function TM decreases the work function of the complex oxide, thereby decreasing its reducibility (deoxidation) in H*. This leads to the stabilization of the higher oxidation states of a high-work function TM, which otherwise would be readily reduced in H*. We propose that the work function serves as a tunable parameter, modulating the interaction of hydrogen with TM compounds.

A feasibility study was conducted into the atomic layer deposition (ALD) of metallic molybdenum from the molybdenum hexacarbonyl [Mo(CO)6] precursor. Without the use of a coreactant, Mo(CO)6 decarbonylates in a nonself-limiting fashion to form molybdenum oxycarbide (MoCxOy) films in the low-temperature regime between 100 and 250 °C. Introducing (atomic) hydrogen as a coreactant, in an attempt to drive self-limiting growth and provide metallic molybdenum films, caused hardly a difference in both film composition and growth kinetics. With ozone as a coreactant, an ALD process was developed to grow molybdenum oxide (MoO3) films without carbon contamination. The MoO3 ALD cycle times were optimized and the existence of an ALD window was investigated. The MoO3 films were subsequently reduced by atomic hydrogen to form metallic molybdenum at temperatures between 150 and 450 °C. The degree of reduction was shown to increase with the reduction temperature, with the limitation that the film exhibited multiple cracks after reduction at 450 °C. Spectroscopic ellipsometry, x-ray photoelectron spectroscopy, and scanning and transmission electron microscopy were employed for thin-film characterization.

Amidst the growing importance of hydrogen in a sustainable future, it is crucial to develop coatings that can protect hydrogen-sensitive system components in reactive hydrogen environments. However, the prediction of the chemical stability of materials in hydrogen is not fully understood. In this study, we show that the work function is a key parameter determining the reducibility (i.e., denitridation) of transition metal nitrides (TMNs) in hydrogen radicals (H*) at elevated temperatures. We demonstrate that, when the work function of a TMN system drops below a threshold limit (phi TH), its reduction effectively stops. We propose that this is due to the preferential binding of H* to transition metal (TM) atoms rather than N atoms, which makes the formation of volatile species (NH x ) unfavorable. This finding provides a novel perspective for comprehending the interaction of hydrogen with TM compounds and allows prediction of the chemical stability of hydrogen-protective coatings.