Victoria Soghomonian

An Inorganic Double Helix: Hydrothermal Synthesis, Structure, and Magnetism of Chiral [(CH3)2NH2]K4[V10O10(H2O)2(OH)4(PO4)7]·4H2O

Very complicated inorganic solids can be self-assembled from structurally simple precursors as illustrated by the hydrothermal synthesis of the vanadium phosphate, [(CH3)2NH2]K4[V10O10(H2O)2(OH)4(PO4)7]�4H2O, 1, which contains chiral double helices formed from interpenetrating spirals of vanadium oxo pentamers bonded together by P5+. These double helices are in turn intertwined with each other in a manner that generates unusual tunnels and cavities that are filled with (CH3)2NH2+ and K+ cations, respectively. The unit cell contents of dark blue phosphate 1, which crystallizes in the enantiomorphic space group P43 with lattice constants a = 12.130 and c = 30.555 angstroms, are chiral; only one enantiomorph is present in a given crystal. Magnetization measurements show that 1 is paramagnetic with ten unpaired electrons per formula unit at higher temperatures and that antiferromagnetic interactions develop at lower temperatures.

Comparative current–voltage characteristics of nicked and repaired λ-DNA

We report on current–voltage characteristics obtained at room temperature on λ-DNA molecules, modified at both ends with disulfide groups and spanning Au electrodes. A comparison is drawn between the characteristics of the λ-DNA, which after modification features a gap between 3′ and 5′ sites (nicked DNA), and λ-DNA where this gap has been ligated (repaired DNA). The repaired DNA double helices show a close-to-linear current–voltage characteristic, and a dc conductivity estimated at ∼1×10−3 S cm−1. In contrast, the nicked DNA shows pronouncedly nonlinear and rectifying behavior, with a conductivity gap of ∼3 eV. The low-field conductivity of the nicked DNA is approximately a factor 20 lower than the repaired DNA’s conductivity.

Gate tunable electron injection in submicron pentacene transistors

We study submicron organic field effect transistors with a pentacene channel, and observe either p-type or n-type behaviour under different gate and drain voltage conditions. Transistor structures of 0.8??m channel lengths were fabricated by evaporating Au on a tilted substrate, featuring an oxide step. When evaporating pentacene on the step structure, the edge of the oxide step is used as a shadow mask to ensure the gap between source and drain. Current?voltage characteristics reveal that positive gate voltages increase the drain current, when the lower Au contact is operated as drain electrode, indicating electron transport through the channel. When the upper Au contact is used as a drain, the devices display p-type behaviour. These ambipolar device characteristics are explained in the light of electron injection enhanced by the submicron geometry, and by electron transport in the presence of electron traps.

High resolution and high fields in biological solid state NMR

2H solid state NMR spectra of a polypeptide in an oriented membrane environment is demonstrated to have an orientational resolution of 0.3 degree. Such data results in high resolution structural constraints. Similar spectra are demonstrated at 23.2 T using a resistive magnet at the National High Magnetic Field Laboratory.

Characterization of electrical conductivity in a zeolitelike material

We present the electrical characterization of a zeolitelike oxo-vanadium arsenate framework. The experimentally obtained electronic and ionic conductivities and their interactions are discussed. Further, we investigate the potential use of electrically conducting zeolitelike materials in electrical energy storage applications, in light of the material’s structural and electronic characteristics.

Hydrothermal synthesis and structural characterization of the three-dimensional framework solids (NH4)[V2(OH)(PO4)2(H2O)]·H2O and Rb6[(Mo9V3O6)(PO4)10(H2PO4)3(OH)9]·8.5H2O

Abstract Hydrothermal synthesis has been employed to prepared two three-dimensional, open-framework solids with tetranuclear metal oxide cores. The reaction of VCl4, H3PO4, 1,3-diaminopropane and H2O in the mole ratio 1:7,4:8.6:1004 at 200°C for 87 h yielded (NH4) [V2(OH)(PO4)2(H2O)]·H2O(1·H2O), while the reaction of Rb2MoO4, Rb2V2O6, vanadium metal, n-Bu4NBr, H3PO4 and H2O in the mole ratio 2:1.1:1:1:145:392 at 240°C for 48 h produced Rb6[(Mo9V3O6)(PO4)10(H2PO4)3(OH)9]·8.5H2O (2·8.5H2O). The structure of 1·H2O is constructed from tetranuclear units of edge- and corner-sharing V(III) octahedral linked into a three-dimensional framework through phosphate tetrahedra. Compound 2·8.5H2O exhibits structurally distinct tetranuclear units of edge- and corner-sharing Mo(V) and Mo(III)/V(III) octahedra linked through phosphate tetrahedra into a open-framework solid with hexagonal channels. Both 1·H2O and 2·8.5H2O exhibit microporosity as evidenced by Type 1 water sorption isotherms. Crystal data: H8O12P2V2 (1·H2O), monoclinic P2 I n , a=9.803(2), b = 9.688(2), c = 9.858(2) A , β = 102.91(3)°, V = 912.6(3) A ; structure solution and refinement converged at 0.0448 for 1481 reflections. H32O75 5P13V3Mo9Rb6 (2·8.5H2O), hexagonal P6 3 m , a = 16.563(2), c = 12.534(3) A , V = 2977.8(7) A 3 ; R = 0.0570 for 3022 reflections.

Quantum interference measurement of spin interactions in a bio-organic/semiconductor device structure

Quantum interference is used to measure the spin interactions between an InAs surface electron system and the iron center in the biomolecule hemin in nanometer proximity in a bio-organic/semiconductor device structure. The interference quantifies the influence of hemin on the spin decoherence properties of the surface electrons. The decoherence times of the electrons serve to characterize the biomolecule, in an electronic complement to the use of spin decoherence times in magnetic resonance. Hemin, prototypical for the heme group in hemoglobin, is used to demonstrate the method, as a representative biomolecule where the spin state of a metal ion affects biological functions. The electronic determination of spin decoherence properties relies on the quantum correction of antilocalization, a result of quantum interference in the electron system. Spin-flip scattering is found to increase with temperature due to hemin, signifying a spin exchange between the iron center and the electrons, thus implying interactions between a biomolecule and a solid-state system in the hemin/InAs hybrid structure. The results also indicate the feasibility of artificial bioinspired materials using tunable carrier systems to mediate interactions between biological entities.

Current–voltage characteristics of diversely disulfide terminated λ-deoxyribonucleic acid molecules

Current–voltage characteristics were obtained at room temperature on duplex λ-deoxyribonucleic acid (DNA) molecules labeled with disulfide end groups in two configurations: Either the disulfide groups were attached to the 3′ ends of opposite DNA strands, or on the same strand at one 3′ end and at one 5′ end. In the latter configuration, only one strand is attached and contacted to the two Au electrodes utilized in the measurement. The current–voltage characteristics of both configurations are linear and do not reveal significant differences. The disulfide end groups in either configuration thus provide an equivalent contact. Moreover, the strand contacted at both ends by disulfide groups is not preferred for charge transport which, hence, involves the complete double-helical structure.

Gallium Arsenate Dihydrate under Pressure: Elastic Properties, Compression Mechanism, and Hydrogen Bonding

Gallium arsenate dihydrate is a member of a class of isostructural compounds, with the general formula M(3+)AsO4·2H2O (M(3+) = Fe, Al, In, or Ga), which are being considered as potential solid-state storage media for the sequestration of toxic arsenic cations. We report the first high-pressure structural analysis of a metal arsenate dihydrate, namely, GaAsO4·2H2O. This compound crystallizes in the orthorhombic space group Pbca with Z = 8. Accurate unit cell parameters as a function of pressure were obtained by high-pressure single-crystal X-ray diffraction, and a bulk modulus of 51.1(3) GPa for GaAsO4·2H2O was determined from a third-order Birch-Murnaghan equation of state fit to the P-V data. Assessment of the pressure dependencies of the unit cell lengths showed that the compressibility of the structure along the axial directions increases in the order of [010] < [100] < [001]. This order was found to correlate well with the proposed compression mechanism for GaAsO4·2H2O, which involves deformation of the internal channel void spaces of the polyhedral helices that lie parallel to the [010] direction, and increased distortion of the GaO6 octahedra. The findings of the high-pressure diffraction experiment were further supported by the results from variable-pressure Raman analysis of GaAsO4·2H2O. Moreover, we propose a revised and more complex model for the hydrogen-bonding scheme in GaAsO4·2H2O, and on the basis of this revision, we reassigned the peaks in the OH stretching regions of previously published Raman spectra of this compound.

Comparative ion insertion study into a nanostructured vanadium oxide in aqueous salt solutions

We present a comparative study for the electrochemical insertion of different cations into a nanostructured vanadium oxide material. The oxide is hydrothemally synthesized and electrically characterized by variable temperature measurements. The electrochemical reactions are performed in aqueous chloride solutions of lithium, sodium, potassium, and ammonium, and the electrochemical behavior of various cycles are correlated with visual changes in the vanadium oxide nanosheets as observed by scanning electron microscopy. We note an increase in the specific charge per cycle in the cases of sodium and ammonium ions only, correlated with minimal physical changes to the nanosheets. The differing behavior of the various ions has implications for their use in electrical energy storage applications.