Publications

We successfully demonstrated the utility of surface science techniques - namely scanning probe microscopy and thermal desorption spectroscopy - on three different material systems: incipient soot formed during fossil fuel combustion, surface oxides passivating polycrystalline nickel hydrogen uptake, and aluminum hydride cluster formation underpinning solid-state hydrogen fuel storage. For all three material systems, surface science techniques haven proven to probe intricate nanoscale phenomena that are critical to macroscale material behavior. This LDRD has gained insight into early-stage pollution formation, the impacts of common contaminants on tritium flow regulation, and the limitations of solid-state hydrogen fuel storage. Our results support the diversification of national energy technologies.

The passivation of polycrystalline nickel surfaces against hydrogen uptake by oxygen is investigated experimentally with low energy ion scattering (LEIS), direct recoil spectroscopy (DRS), and thermal desorption spectroscopy (TDS). These techniques are highly sensitive to surface hydrogen, allowing the change in hydrogen adsorption in response to varying amounts of oxygen exposure to be measured. The chemical composition of a nickel surface during a mixed oxygen and hydrogen exposure was characterized with LEIS and DRS, while the uptake and activation energies of hydrogen on a nickel surface with preadsorbed oxygen were quantified with TDS. By and large, these measurements of how the oxygen and hydrogen surface coverage varied in response to oxygen exposure were found to be consistent with predictions of a simple site-blocking model. This finding suggests that, despite the complexities that arise due to polycrystallinity, the oxygen-induced passivation of a polycrystalline nickel surface against hydrogen uptake can be approximated by a simple site-blocking model.

Metal oxides have been an attractive option for a range of applications, including hydrogen sensors, microelectronics, and catalysis, due to their reactivity and tunability. The properties of metal oxides can vary greatly on their precise surface structure; however, few surface science techniques can achieve atomistic-level determinations of surface structure, and fewer yet can do so for insulator surfaces. Low energy ion beam analysis offers a potential insulator-compatible solution to characterizing the surface structure of metal oxides. As a feasibility study, we apply low energy ion beam analysis to investigate the surface structure of a magnetite single crystal, Fe₃O₄(100). We obtain multi-angle maps using both forward-scattering low energy ion scattering (LEIS) and backscattering impact-collision ion scattering spectroscopy (ICISS). Both sets of experimental maps have intensity patterns that reflect the symmetries of the Fe₃O₄(100) surface structure. However, analytical interpretation of these intensity patterns to extract details of the surface structure is significantly more complex than previous LEIS and ICISS structural studies of one-component metal crystals, which had far more symmetries to exploit. To gain further insight into the surface structure, we model our experimental measurements with ion-trajectory tracing simulations using molecular dynamics. Our simulations provide a qualitative indication that our experimental measurements agree better with a subsurface cation vacancy model than a distorted bulk model.

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We have used scanning tunneling microscopy and density functional theory calculations to study molecular layers of coronene on Cu(111). The structure and stability of these layers is determined by the balance between coronene-substrate and coronene-coronene interactions. Here, we characterize this balance by measuring the maximum coverage before coronene dewets the substrate to form three-dimensional islands. We find that coronene molecules lie parallel to the substrate at the maximum coverage, in contrast to previous observations of tilted coronene on metal surfaces. We attribute this previously reported tilt to a metastability caused by an activation barrier to nucleate three-dimensional islands.

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