Elena Echeverria

A New Approach to Non-Coordinating Anions: Lewis Acid Enhancement of Porphyrin Metal Centers in a Zwitterionic Metal-Organic Framework.

We describe a new strategy to generate non-coordinating anions using zwitterionic metal-organic frameworks (MOFs). By assembly of anionic inorganic secondary building blocks (SBUs) ([In(CO2)4](-)) with cationic metalloporphyrin-based organic linkers, we prepared zwitterionic MOFs in which the complete internal charge separation effectively prevents the potential binding of the counteranion to the cationic metal center. We demonstrate the enhanced Lewis acidity of Mn(III)- and Fe(III)-porphyrins in the zwitterionic MOFs in three representative electrocyclization reactions: [2 + 1] cycloisomerization of enynes, [3 + 2] cycloaddition of aziridines and alkenes, and [4 + 2] hetero-Diels-Alder cycloaddition of aldehydes with dienes. This work paves a new way to design functional MOFs for tunable chemical catalysis.

Use of thiolated oligonucleotides as anti-fouling diluents in electrochemical peptide-based sensors.

We incorporated short thiolated oligonucleotides as passivating diluents in the fabrication of electrochemical peptide-based (E-PB) sensors, with the goal of creating a negatively charged layer capable of resisting non-specific adsorption of matrix contaminants. The E-PB HIV sensors fabricated using these diluents were found to be more specific and selective, while retaining attributes similar to the sensor fabricated without these diluents. Overall, these results highlight the advantages of using oligonucleotides as anti-fouling diluents in self-assembled monolayer-based sensors.

Semiconducting boron carbides with better charge extraction through the addition of pyridine moieties

The plasma-enhanced chemical vapor (PECVD) co-deposition of pyridine and 1,2 dicarbadodecaborane, 1,2-B10C2H12 (orthocarborane) results in semiconducting boron carbide composite films with a significantly better charge extraction than plasma-enhanced chemical vapor deposited semiconducting boron carbide synthesized from orthocarborane alone. The PECVD pyridine/orthocarborane based semiconducting boron carbide composites, with pyridine/orthocarborane ratios ~3:1 or 9:1 exhibit indirect band gaps of 1.8 eV or 1.6 eV, respectively. These energies are less than the corresponding exciton energies of 2.0 eV–2.1 eV. The capacitance/voltage and current/voltage measurements indicate the hole carrier lifetimes for PECVD pyridine/orthocarborane based semiconducting boron carbide composites (3:1) films of ~350 µs compared to values of ≤35 µs for the PECVD semiconducting boron carbide films fabricated without pyridine. The hole carrier lifetime values are significantly longer than the initial exciton decay times in the region of ~0.05 ns and 0.27 ns for PECVD semiconducting boron carbide films with and without pyridine, respectively, as suggested by the time-resolved photoluminescence. These data indicate enhanced electron–hole separation and charge carrier lifetimes in PECVD pyridine/orthocarborane based semiconducting boron carbide and are consistent with the results of zero bias neutron voltaic measurements indicating significantly enhanced charge collection efficiency.

Band structure characterization of WS2 grown by chemical vapor deposition

Growth by chemical vapor deposition (CVD) leads to multilayer WS2 of very high quality, based on high-resolution angle-resolved photoemission spectroscopy. The experimental valence band electronic structure is considered to be in good agreement with that obtained from density functional theory calculations. We find the spin-orbit splitting at the K¯ point to be 420  ± 20 meV with a hole effective mass of −0.35  ± 0.02 me for the upper spin-orbit component (the branch closer to the Fermi level) and −0.43  ± 0.07 me for the lower spin-orbit component. As predicted by theory, a thickness-dependent increase of bandwidth is observed at the top of the valence band, in the region of the Brillouin zone center. The top of the valence band of the CVD-prepared films exhibits a substantial binding energy, consistent with n-type behavior, and in agreement with transistor characteristics acquired using devices incorporating the same WS2 material.

Increased drift carrier lifetime in semiconducting boron carbides deposited by plasma enhanced chemical vapor deposition from carboranes and benzene

Plasma-enhanced chemical vapor (PECVD) codeposition of benzene and 1,2-dicarbadodecaborane, 1,2-B10C2H12 (orthocarborane) and benzene, 1,7 dicarbadodecaborane, and 1,7-B10C2H12 (metacarborane) results in semiconducting boron carbide composite films with significantly longer drift carrier lifetimes than plasma-enhanced chemical vapor deposited semiconducting boron carbide synthesized from orthocarborane or metacarborane alone. Capacitance versus voltage C ( V ) and current versus voltage I ( V ) measurements indicate the hole carrier lifetimes for PECVD benzene/orthocarborane based semiconducting boron carbide composites increase to 2.5 ms from values of ≤35 μs for the PECVD semiconducting boron carbide films fabricated without benzene. For PECVD benzene/metacarborane based semiconducting boron carbide composites, there is an increase in the hole carrier lifetime to roughly 300 ns from values of 50 ns for those films fabricated without benzene.

Electronic structure of cyclodextrin–carbon nanotube composite films

The electronic structures of two kinds of cyclodextrin–carbon nanotube (αCD–CNT and γCD–CNT) composite films are investigated by using (angular dependent) photoelectron spectroscopy to gain insight as to why the αCD–CNT and γCD–CNT composite films show different performances in biosensor applications. The γCD–CNT composite film is likely to have the CD localized on the surface rather than in the bulk of the film, while αCD–CNT has CD relatively more concentrated within the bulk of selvedge region of the film, rather than the surface. The results indicate that the CD, of the γCD–CNT composite, may be more bioactive, and possibly a better sensor of biomolecules due to the favorable surface position compared with that of αCD–CNT. The valence band of αCD–CNT and γCD–CNT show little difference from the CNT film except for a density of states, originating from CD, evident at a binding energy near 27 eV below Fermi level, meaning that there are few or no redox interactions between the CD and the CNT. The absence of a redox interaction between the CD and the CNT permits a clear electrochemical response to occur when guest biomolecules are captured on the composites, providing a route to biosensor applications.

Band Bending at the Gold (Au)/Boron Carbide-Based Semiconductor Interface

Abstract We have used X-ray photoemission spectroscopy to study the interaction of gold (Au) with novel boron carbide-based semiconductors grown by plasma-enhanced chemical vapor deposition (PECVD). Both n- and p-type films have been investigated and the PECVD boron carbides are compared to those containing aromatic compounds. In the case of the p-type semiconducting PECVD hydrogenated boron carbide samples, the binding energy of the B(1s) core level shows a shift to higher binding energies as the Au is deposited, an indication of band bending and possibly Schottky barrier formation. In the case of the n-type boron carbide semiconductors the interaction at the interface is more typical of an ohmic contact. Addition of the aromatic compounds increases the change in binding energies on both n-type and p-type PECVD boron carbide semiconductors, and the gold appears to diffuse into the PECVD boron carbides alloyed with aromatic moieties.

Critical-point model dielectric function analysis of WO3 thin films deposited by atomic layer deposition techniques

WO3 thin films were grown by atomic layer deposition and spectroscopic ellipsometry data gathered in the photon energy range of 0.72–8.5 eV, and from multiple samples were utilized to determine the frequency dependent complex-valued isotropic dielectric function for WO3. We employ a critical-point model dielectric function analysis and determine a parameterized set of oscillators and compare the observed critical-point contributions with the vertical transition energy distribution found within the band structure of WO3 calculated by the density functional theory. The surface roughness was investigated using atomic force microscopy, and compared with the effective roughness as seen by the spectroscopic ellipsometry.