Publications

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The earliest stages of soot formation in flames are believed to involve the formation of small, nanoscale clusters of polycyclic aromatic hydrocarbon molecules. The structure of these clusters is still highly uncertain, however, impeding the construction of quantitative models of soot inception and growth. To provide insight into the structure of incipient soot, we produced nanoclusters of hydrocarbon molecules by annealing coronene films deposited on Pt(111), and examined them with scanning tunneling microcopy. We find that clusters containing ∼20–100 molecules, are disordered agglomerations of stacks that are ∼5–6 molecules tall. These structures are quite distinct from crystalline coronene, but bear a striking resemblance to recently proposed models for the equilibrium structure of similarly-sized clusters that are assumed to initiate soot formation. In contrast to mature soot, the surfaces of these clusters contain very few molecules with graphitic planes oriented parallel to the surface.

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Next generation metal-ion conducting membranes are key to developing energy storage and utilization technologies like batteries and fuel ce lls. Sodium super-ionic conductors (aka NaSICON) are a class of compounds with AM 1 M 2 (PO 4 ) 3 stoichiometry where the choice of "A" and "M" cation varies widely. This report, which de scribes substitutional derivatives of NZP (NaZr 2 P 3 O 12 ), summarizes the accomplishments of a Laboratory D irected Research and Development (LDRD) project to analyze transport mec hanisms using a combination of in situ studies of structure, composition, and bonding, com bined with first principles theory and modeling. We developed an experimental platform and applied methods, such as synchrotron- based X-ray spectroscopies, to probe the electronic structure of compositionally well-controlled NaSICON films while in operation ( i.e ., conducting Na ions exposed to oxygen or water va por atmospheres). First principles theory and modeling were used to interpret the experimental observations and develop an enhanced understanding of atomistic processes that give rise to, and affect, ion conduction.

Carbon nanostructures, such as nanotubes and graphene, are of considerable interest due to their unique mechanical and electrical properties. The materials exhibit extremely high strength and conductivity when defects created during synthesis are minimized. Atomistic modeling is one technique for high resolution studies of defect formation and mitigation. To enable simulations of the mechanical behavior and growth mechanisms of C nanostructures, a high-fidelity analytical bond-order potential for the C is needed. To generate inputs for developing such a potential, we performed quantum mechanical calculations of various C structures.

The electrochemical reactions of solid oxide fuel cells occur in the region where gas-phase species, electrode, and electrolyte coincide. When the electrode is an ionic insulator and the electrolyte is an electronic insulator, this triple phase boundary is assumed to have atomic dimensions. Here we use photoemission electron microscopy to show that the reduced surface of the electrolyte yttria-stabilized zirconia (YSZ) has a sharp electronic metal-insulator boundary near Pt negative electrodes. The electronic conductivity of the reduced YSZ allows for oxygen reduction, allowing the reduced YSZ to behave as an extended triple phase boundary. This extended triple phase boundary can be many microns in size, depending on oxygen pressure, temperature, applied voltage, and time.

In this project we developed t he atomistic models needed to predict how graphene grows when carbon is deposited on metal and semiconductor surfaces. We first calculated energies of many carbon configurations using first principles electronic structure calculations and then used these energies to construct an empirical bond order potentials that enable s comprehensive molecular dynamics simulation of growth. We validated our approach by comparing our predictions to experiments of graphene growth on Ir, Cu and Ge. The robustness of ou r understanding of graphene growth will enable high quality graphene to be grown on novel substrates which will expand the number of potential types of graphene electronic devices.

Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, â €- ripeningâ €™; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1â €‰meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moiré corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials.

The atomistic mechanism for lithiation/delithiation in all-solid-state batteries is still an open question, and the 'holy grail' to engineer devices with extended lifetime. Here, by combining real-time scanning electron microscopy in ultra-high vacuum with electrochemical cycling, we quantify the dynamic degradation of Al anodes in Li-ion all-solid-state batteries, a promising alternative for ultra lightweight devices. We find that AlLi alloy mounds are formed on the top surface of the Al anode and that degradation of battery capacity occurs because of Li trapped in them. Our approach establishes a new platform for probing the real-time degradation of electrodes, and can be expanded to other complex systems, allowing for high throughput characterization of batteries with nanoscale resolution.

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