Publications

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Organic linkers in metal-organic framework (MOF) materials exhibit differences in hydrogen bonding (H-bonding), which can alter the geometric, electronic, and optical properties of the MOF. Density functional theory (DFT) simulations were performed on a photoluminescent Y-2,5-dihydroxyterephthalic acid (DOBDC) MOF with H-bonding concentrations between 0 and 100%; the H-bonds were located on both bidentate-and monodentate-bound DOBDC linkers. At 0% H-bond concentration in the framework, the lattice parameters contracted, the density increased, and simulated X-ray diffraction patterns shifted. Comparison with published experimental data identified that Y-DOBDC MOF structures must have a degree of H-bond concentration. The concentration of H-bonds in the system shifted the calculated band gap energy from 2.25 eV at 100% to 3.00 eV at 0%. The band gap energies also indicate a distinction of H-bonds formed on bidentate-coordinated linkers compared to those on monodentate linkers. Additionally, when the calculated optical spectra are compared with experimental data, the ligand-to-ligand charge-transfer luminescence in Y-DOBDC MOFs is expected to result from an average of 20-40% H-bonding with at least 50% of the bidentate linkers containing H-bonding. Therefore, the type of H-bonding within the DOBDC linker determines the electronic structure and the optical absorption of the MOF framework structure. Tuning of the H-bonding in rare-earth MOFs provides an opportunity to control the specific optical and adsorption properties of the MOF framework on the basis of reactions between the linker and the environment.

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Herein is presented the synthesis and characterization of copper-intercalated zirconium pentatelluride (ZrTe₅). ZrTe₅:Cu_0.05 crystals are synthesized by the chemical vapor transport method in a vacuum. X-ray diffraction and elemental analysis techniques are utilized to validate the synthesis. The results indicate that the intercalation of the layered Zr/Te structure with copper atoms causes the contraction of the unit cell along all three crystalline directions, the shrinkage of the overall volume of the unit cell, and the distortion of the unit cell. A single crystal was isolated, mechanically exfoliated, and used for the measurements of intercalation strains in a Hall bar device. Electronic transport studies indicate that an anomalous resistance drop is observed at T = 19 K. Furthermore, R_xx and R_xy results, respectively, indicate a probable disorder-induced localization effect and electron-type carriers.

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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.

Recent years have seen an explosion in research efforts discovering and understanding novel electronic and optical properties of topological quantum materials (TQMs). In this LDRD, a synergistic effort of materials growth, characterization, electrical-magneto-optical measurements, combined with density functional theory and modeling has been established to address the unique properties of TQMs. Particularly, we have carried out extensive studies in search for Majorana fermions (MFs) in TQMs for topological quantum computation. Moreover, we have focused on three important science questions. 1) How can we controllably tune the properties of TQMs to make them suitable for quantum information applications? 2) What materials parameters are most important for successfully observing MFs in TQMs? 3) Can the physical properties of TQMs be tailored by topological band engineering? Results obtained in this LDRD not only deepen our current knowledge in fundamental quantum physics but also hold great promise for advanced electronic/photonic applications in information technologies.

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Iodine detection is crucial for nuclear waste clean-up and first responder activities. For ease of use and durability of response, robust active materials that enable the direct electrical detection of I2 are needed. Herein, a large reversible electrical response is demonstrated as I2 is controllably and repeatedly adsorbed and desorbed from a series of metal-organic frameworks (MOFs) MFM-300(X), each possessing a different metal center (X = Al, Fe, In, or Sc) bridged by biphenyl-3,3′,5,5′-tetracarboxylate linkers. Impedance spectroscopy is used to evaluate how the different metal centers influence the electrical response upon cycling of I2 gas, ranging from 10× to 106× decrease in resistance upon I2 adsorption in air. This large variation in electrical response is attributed not only to the differing structural characteristics of the MOFs but also to the differing MOF morphologies and how this influences the degree of reversibility of I2 adsorption. Interestingly, MFM-300(Al) and MFM-300(In) displayed the largest changes in resistance (up to 106×) yet lost much of their adsorption capacity after five I2 adsorption cycles in air. On the other hand, MFM-300(Fe) and MFM-300(Sc) revealed more moderate changes in resistance (10-100×), maintaining most of their original adsorption capacity after five cycles. This work demonstrates how changes in MOFs can profoundly affect the magnitude and reversibility of the electrical response of sensor materials. Tuning both the intrinsic (resistivity and adsorption capacity) and extrinsic (surface area and particle morphology) properties is necessary to develop highly reversible, large signal-generating MOF materials for direct electrical readout for I2 sensing.

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