Publications Search

HfC has shown promise as a material for field emission due to the low work function of the (100) surface and a high melting point. Recently, HfC tips have exhibited unexpected failure after field emission at 2200 K. Characterization of the HfC tips identified faceting of the parabolic tip dominated by coexisting (100) and (111) surfaces. To investigate this phenomenon, we used density functional theory (DFT) simulations to identify the role of defects and impurities (Ta, N, O) on HfC surface properties. Carbon vacancies increased the surface energy of the (100) surface from 2.35 J/m2 to 4.75 J/m2 and decreased the surface energy of the carbon terminated (111) surface from 8.75 J/m2 to 3.48 J/m2. Once 60% of the carbon on the (100) surface have been removed the hafnium terminated (111) surface becomes the lowest energy surface, suggesting that carbon depletion may cause these surfaces to coexist. The addition of Ta and N impurities to the surface are energetically favorable and decrease the work function, making them candidate impurities for improving field emission at high temperatures. Overall, DFT simulations have demonstrated the importance of understanding the role of defects on the surface structure and properties of HfC.

More Details

TYPE Journal Article YEAR 2019

DOI OSTI Scopus

Structural and Optical Response of Complex Mixed Gas Dynamics in Rare Earth Metal-Organic Frameworks

Vogel, Dayton J.; Henkelis, Susan E.; Nenoff, T.M.; Rimsza, Jessica R.

Abstract not provided.

More Details

TYPE Conference Poster YEAR 2019

OSTI

NO_x Adsorption and Optical Detection in Rare Earth Metal–Organic Frameworks

ACS Applied Materials and Interfaces

Nenoff, T.M.; Vogel, Dayton J.; Rimsza, Jessica R.; Sava Gallis, Dorina F.; Garibay, Grace A.

Acid gases (e.g., NO_x and SO_x), commonly found in complex chemical and petrochemical streams, require material development for their selective adsorption and removal. Here, we report the NO_x adsorption properties in a family of rare earth (RE) metal–organic frameworks (MOFs) materials. Fundamental understanding of the structure–property relationship of NO_x adsorption in the RE-DOBDC materials platform was sought via a combined experimental and molecular modeling study. No structural change was noted following humid NO_x exposure. Density functional theory (DFT) simulations indicated that H₂O has a stronger affinity to bind with the metal center than NO₂, while NO₂ preferentially binds with the DOBDC ligands. Further modeling results indicate no change in binding energy across the RE elements investigated. Also, stabilization of the NO₂ and H₂O molecules following adsorption was noted, predicted to be due to hydrogen bonding between the framework ligands and the molecules and nanoconfinement within the MOF structure. This interaction also caused distinct changes in emission spectra, identified experimentally. As a result, calculations indicated that this is due to the adsorption of NO₂ molecules onto the DOBDC ligand altering the electronic transitions and the resulting photoluminescent properties, a feature that has potential applications in future sensing technologies.