Helena Grennberg

Defect formation in graphene nanosheets by acid treatment: an x-ray absorption spectroscopy and density functional theory study

In-plane defects have been introduced into graphene nanosheets by treatment with hydrochloric acid. Acid treatment induces bond cleavage in the C–C network via electrophilic attack. These resultant vacancy sites will then undergo further reactions with the surrounding ambient to produce C–O and C–H bonds. A σ ∗ resonance at 287 eV in the carbon K-edge x-ray absorption spectra is observed with acid treatment and is assigned to C–O states. Theoretical modelling of a di-vacancy in a graphene bilayer reproduces all essential features of this resonance and in addition predicts a metallic conductivity of states around this vacancy. The possibility of engineering the properties of graphene via the routes explored here is an important step towards establishing strategies for building devices based on this material. (Some figures in this article are in colour only in the electronic version)

Conductivity engineering of graphene by defect formation

Transport measurements have revealed several exotic electronic properties of graphene. The possibility to influence the electronic structure and hence control the conductivity by adsorption or doping with adatoms is crucial in view of electronics applications. Here, we show that in contrast to expectation, the conductivity of graphene increases with increasing concentration of vacancy defects, by more than one order of magnitude. We obtain a pronounced enhancement of the conductivity after insertion of defects by both quantum mechanical transport calculations as well as experimental studies of carbon nano-sheets. Our finding is attributed to the defect induced mid-gap states, which create a region exhibiting metallic behaviour around the vacancy defects. The modification of the conductivity of graphene by the implementation of stable defects is crucial for the creation of electronic junctions in graphene-based electronics devices.

Mild sonochemical exfoliation of bromine-intercalated graphite: a new route towards graphene

A method to produce suspensions of graphene sheets by combining solution-based bromine intercalation and mild sonochemical exfoliation is presented. Ultrasonic treatment of graphite in water leads ...

Mechanism of palladium-catalyzed allylic acetoxylation of cyclohexene

The “oxypalladation route” or the “(π-allyl)palladium route”? The mechanism of the quinone-based palladium-catalyzed allylic oxidation of olefins to allylic carboxylates (shown schematically below) has been under debate for some time. Isotopically labeled substrates have been used to provide evidence for the “(π-allyl)palladium route” in the allylic acetoxylation of cyclohexene.

Aerobic palladium-heteropolyacid-catalyzed allylic acetoxylation of cyclohexene

The heteropolyacid H5[PMo10V2O40] is an effective oxygen-activating catalyst in palladium-catalyzed aerobic allylic acetoxylation of cyclohexene.

How Close Can You Get? Studies of Ultrafast Light-Induced Processes in Ruthenium-[60] Fullerene Dyads with Short Pyrazolino and Pyrrolidino Links

Two pyrazoline- and one pyrrolidine-bridged Ru(II)bipyridine-[60]fullerene dyads have been prepared and studied by ultrafast time-resolved spectroscopy. A silver-assisted synthesis route, in which Ag(I) removes the chlorides from the precursor complex Ru(bpy) 2Cl 2 facilitates successful coordination of the [60]fullerene-substituted third ligand. Upon light excitation of the ruthenium moiety, the emission was strongly quenched by the fullerene. The main quenching mechanism is an exceptionally fast direct energy transfer ( k obs > 1 x 10 (12) s (-1) in the pyrazoline-bridged dyads), resulting in population of the lowest excited triplet state of fullerene. No evidence for electron transfer was found, despite the extraordinarily short donor-acceptor distance that could kinetically favor that process. The observations have implications on the ongoing development of devices built from Ru-polypyridyl complexes and nanostructured carbon, such as C 60 or nanotubes.

A versatile photoelectron spectrometer for pressures up to 30 mbar

High-pressure photoelectron spectroscopy is a rapidly developing technique with applications in a wide range of fields ranging from fundamental surface science and catalysis to energy materials, environmental science, and biology. At present the majority of the high-pressure photoelectron spectrometers are situated at synchrotron end stations, but recently a small number of laboratory-based setups have also emerged. In this paper we discuss the design and performance of a new laboratory based high pressure photoelectron spectrometer equipped with an Al Kα X-ray anode and a hemispherical electron energy analyzer combined with a differentially pumped electrostatic lens. The instrument is demonstrated to be capable of measuring core level spectra at pressures up to 30 mbar. Moreover, valence band spectra of a silver sample as well as a carbon-coated surface (graphene) recorded under a 2 mbar nitrogen atmosphere are presented, demonstrating the versatility of this laboratory-based spectrometer.

Stirring-induced aggregation of graphene in suspension

Graphene in suspension undergoes stirring-induced aggregation that leads to reversible agglomeration and folding/scrolling, all of which affects the Raman spectra; the findings are of importance in all solution-based protocols for graphene preparation and processing.

Evidence for a (π-allyl)palladium intermediate in the quinone-based palladium-catalysed allylic acetoxylation

The mechanism of the quinone-based palladium-catalysed allylic acetoxylation of cyclohexene is studied using 1,2-dideuteriocyclohexene (55–70% D) as substrate; the distribution of the deuterium label in the product, determined by 1H NMR spectroscopy, is that expected for a (π-allyl)palladium intermediate.

Structure and reactivity of cis- and trans-bis-[{5-carbomethoxy-(1,2,3-η)-cyclohexenyl}palladium]. Evidence for a (σ-allyl)palladium intermediate in the cis-migration of acetate from palladium to coordinated π-allyl

The mode of attack by acetate on a (π-allyl)palladium complex depends not only on the ligands on palladium, but also on the structure of the complexes; for the bis-[{5-carbomethoxy-(1,2,3-η3)-cyclohexenyl}palladium] complexes, the structures of which were determined by X-ray crystallography and NMR spectroscopy, the cis isomer preferably reacted by external attack whereas the trans isomer, under identical conditions, preferred internal migration.