Graphene based heterostructures

The two dimensional charge carriers in monolayer and bilayer graphene are described by massless and massive chiral Dirac Hamiltonians, respectively. These two-dimensional materials are predicted to exhibit a wide range of behavior, etc. However, graphene devices on a typical three-dimensional insulating substrates such as SiO₂ are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. We have developed a novel technique for substrate engineering of graphene devices using layered dielectric materials to build graphene based vertical heterostructures. We employ hBN, an insulating isomorph of graphite, as a substrate and gate dielectric for graphene electronics. In this review, we describe the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal hBN substrates, using a mechanical transfer process. Graphene devices on hBN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO₂. We use the enhanced mobility of electrons in hBN supported graphene to investigate the effects of electronic interactions. We find that interactions drive spontaneous breaking of the emergent SU(4) symmetry of the graphene Landau levels, leading to a variety of non-trivial integer and fractional quantum Hall states. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.     

Synergistic induction of polypeptides by tumor necrosis factor and interferon-gamma in cells sensitive or resistant to tumor necrosis factor: assessment by computer based analysis of two-dimensional gels using the PDQUEST system

Tumor necrosis factor (TNF) synergistically enhanced the antiproliferative activity of interferon-gamma (IFN-gamma) in both TNF-sensitive and TNF-resistant variants of the cervical carcinoma line, ME-180. TNF alone had no apparent effect on the levels of synthesis of individual proteins in either of these variant cell lines as assessed by computerized two-dimensional gel analysis of cell lysates using the PDQUEST system. However, IFN-gamma enhanced the levels of 18 polypeptides and suppressed the levels of 10 polypeptides in both cell lines. When used in combination in both cell lines, TNF and IFN-gamma induced the synthesis of 10 polypeptides that were not induced by either agent alone. These synergistically induced polypeptides may be crucial to the mechanism of the synergistic antiproliferative action of TNF and IFN-gamma in ME-180 cells.     

Imino-bridged heterocycles. III . Synthesis of mono-bridgehead substituted 11H-benzo[5,6]cyclohepta[1,2-c]pyridin-6,11-imines by regiospecific amine to olefin cyclizations

Employing regiospecific amine to endocyclic olefin and hydroxylamine to exocyclic olefin cyclizations, we have developed pathways to two isomeric, bridgehead methyl-substituted benzo[5,6]cyclohepta[1,2-c]pyridin-6,11-imines. Acid catalyzed cyclization of 5 to 6 was accomplished under exceedingly mild conditions (silica gel/chloroform), whereas, thermal cyclization of 20 to generate 21 was performed analogously to the dibenzo[a,d]cyclohepten-5,10-imine system.     

The2 E↔4 A 2 transition in tetragonal Cr3+ complexes

The² E⁴ A ₂ absorption and emission spectra of [Cr(NH₃)₅(NO₃)](NO₃)₂, [Cr(NH₃)₅(NO₂)] ·(NO₃)₂, and [Cr(NH₃)₅(H₂O)](NO₃)₃ microcrystals have been recorded at 77°K. Tetragonal²E splittings are 209, 188 and 87 cm^–1, respectively. An analysis of the limited vibronic structure has been made and compared to the results for the parent octahedral complex, [Cr(NH₃)₆](NO₃)₃. Vibrations of approximately 270 and 700 cm^–1 are prominent.

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Zusammenfassung Die² E⁴ A ₂ Absorptions- und Emissionsspektren von [Cr(NH₃)₅(NO₃)](NO₃)₂-, [Cr(NH₃)₅ (NO₂)](NO₃)₂- und [Cr(NH₃)₅(H₂O)](NO₃)₃-Mikrokristallen werden für 77° angegeben. Die tetragonalen² E Aufspaltungen sind209, 188 bzw. 87 cm^–1. Eine Analyse der begrenzten vibronischen Struktur wurde vorgenommen und mit den Resultaten für den oktaedrischen Stammkomplex, [Cr(NH₃)₆](NO₃)₃ verglichen. Schwingungen von etwa 270 und 700 cm^–1 treten besonders hervor.

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Résumé Les spectres d'émission et d'absorption² E⁴ A ₂ de microcristaux de [Cr(NH₃)₅(NO₃)](NO₃)₂, [Cr(NH₃)₅(NO₂)](NO₃)₂ et [Cr(NH₃)₅(H₂O)](NO₃)₃ ont été enregistrés à 77° K. Les écartements tétragonaux² E sont respectivement 209, 188 et 87 cm^–1. Une analyse de la structure vibronique limitée a été effectuée et comparée aux résultats pour le complexe octaédral parent [Cr(NH₃)₆](NO₃)₃. Les vibrations au voisinage de 270 et 700 cm^–1 émergent.

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This contribution is dedicated to the memory of Prof. Hans-Ludwig Schläfer, a stimulating colleague and valued friend.     

Isolation of a Dodecanuclear Polyoxo Cluster {Nb12O21} Showing a Rare Case of Five-fold Coordinated Niobium(V) Centers with a Square Pyramidal Geometry

The reactivity of the complexing anthracene-9-carboxylate ligand has been investigated with a niobium(IV) tetrachloride precursor (NbCl₄ ⋅ 2THF) in isopropanol solvent. This resulted in the crystallization of a molecular assembly containing two distinct {Nb₁₂O₂₁} cores surrounded by multiple isopropanolate and anthracenoate ligands. The compound is formulated [Nb₁₂₍(μ₃-O)₃(μ-O)₁₈(C₁₅H₉O₂)₈(OⁱPr)₁₀] ⋅ [Nb₁₂(μ₃-O)₂(μ-O)₁₉(C₁₅H₉O₂)₈(OⁱPr)₁₀] illustrating the two different dodecameric oxo-clusters, for which the niobium(IV) precursor was oxidized in the niobium(V) state during the reactional process. The two distinct {Nb₁₂O₂₁} units mainly differs by the environment of the niobium centers, which exhibits unexpected five-fold coordination (square pyramid) for some of them, together with the classical six-fold coordination (octahedron) as usually found for niobium(V). In the crystallization process, the. IR spectroscopy was used to analyze the esterification reaction occurring between the anthracene acid an isopropanolate ligands responsible of the production of water used in the oxo-condensation of the niobium centers. ⁹³Nb Solid state NMR was tentatively used to assess the occurrence of the different niobium environments.