N. Bovet

Characterization of Eyjafjallajökull volcanic ash particles and a protocol for rapid risk assessment

On April 14, 2010, when meltwaters from the Eyjafjallajökull glacier mixed with hot magma, an explosive eruption sent unusually fine-grained ash into the jet stream. It quickly dispersed over Europe. Previous airplane encounters with ash resulted in sandblasted windows and particles melted inside jet engines, causing them to fail. Therefore, air traffic was grounded for several days. Concerns also arose about health risks from fallout, because ash can transport acids as well as toxic compounds, such as fluoride, aluminum, and arsenic. Studies on ash are usually made on material collected far from the source, where it could have mixed with other atmospheric particles, or after exposure to water as rain or fog, which would alter surface composition. For this study, a unique set of dry ash samples was collected immediately after the explosive event and compared with fresh ash from a later, more typical eruption. Using nanotechniques, custom-designed for studying natural materials, we explored the physical and chemical nature of the ash to determine if fears about health and safety were justified and we developed a protocol that will serve for assessing risks during a future event. On single particles, we identified the composition of nanometer scale salt coatings and measured the mass of adsorbed salts with picogram resolution. The particles of explosive ash that reached Europe in the jet stream were especially sharp and abrasive over their entire size range, from submillimeter to tens of nanometers. Edges remained sharp even after a couple of weeks of abrasion in stirred water suspensions.

A high-mobility two-dimensional electron gas at the spinel/perovskite interface of γ-Al2O3/SrTiO3.

The discovery of two-dimensional electron gases at the heterointerface between two insulating perovskite-type oxides, such as LaAlO(3) and SrTiO(3), provides opportunities for a new generation of all-oxide electronic devices. Key challenges remain for achieving interfacial electron mobilities much beyond the current value of approximately 1,000 cm(2) V(-1) s(-1) (at low temperatures). Here we create a new type of two-dimensional electron gas at the heterointerface between SrTiO(3) and a spinel γ-Al(2)O(3) epitaxial film with compatible oxygen ions sublattices. Electron mobilities more than one order of magnitude higher than those of hitherto-investigated perovskite-type interfaces are obtained. The spinel/perovskite two-dimensional electron gas, where the two-dimensional conduction character is revealed by quantum magnetoresistance oscillations, is found to result from interface-stabilized oxygen vacancies confined within a layer of 0.9 nm in proximity to the interface. Our findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices.

Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces by charge-transfer-induced modulation doping.

Two-dimensional electron gases (2DEGs) formed at the interface of insulating complex oxides promise the development of all-oxide electronic devices. These 2DEGs involve many-body interactions that give rise to a variety of physical phenomena such as superconductivity, magnetism, tunable metal-insulator transitions and phase separation. Increasing the mobility of the 2DEG, however, remains a major challenge. Here, we show that the electron mobility is enhanced by more than two orders of magnitude by inserting a single-unit-cell insulating layer of polar La(1-x)Sr(x)MnO3 (x = 0, 1/8, and 1/3) at the interface between disordered LaAlO3 and crystalline SrTiO3 produced at room temperature. Resonant X-ray spectroscopy and transmission electron microscopy show that the manganite layer undergoes unambiguous electronic reconstruction, leading to modulation doping of such atomically engineered complex oxide heterointerfaces. At low temperatures, the modulation-doped 2DEG exhibits Shubnikov-de Haas oscillations and fingerprints of the quantum Hall effect, demonstrating unprecedented high mobility and low electron density.

Solution‐Processed Ultrathin Chemically Derived Graphene Films as Soft Top Contacts for Solid‐State Molecular Electronic Junctions

A novel method using solution-processed ultrathin chemically derived graphene films as soft top contacts for the non-destructive fabrication of molecular junctions is demonstrated. We believe this protocol will greatly enrich the solid-state test beds for molecular electronics due to its low-cost, easy-processing and flexible nature.

Ultrathin Reduced Graphene Oxide Films as Transparent Top‐Contacts for Light Switchable Solid‐State Molecular Junctions

A new type of solid-state molecular junction is introduced, which employs reduced graphene oxide as a transparent top contact that permits a self-assembled molecular monolayer to be photoswitched in situ, while simultaneously enabling charge-transport measurements across the molecules. The electrical switching behavior of a less-studied molecular switch, dihydroazulene/vinylheptafulvene, is described, which is used as a test case.

Creation of High Mobility Two-Dimensional Electron Gases via Strain Induced Polarization at an Otherwise Nonpolar Complex Oxide Interface

The discovery of two-dimensional electron gases (2DEGs) in SrTiO3-based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO3/SrTiO3. Remarkably, this heterointerface is atomically sharp and exhibits a high electron mobility exceeding 60,000 cm(2) V(-1) s(-1) at low temperatures. The 2DEG carrier density exhibits a critical dependence on the film thickness, in good agreement with the polarization induced 2DEG scheme.

Room Temperature Formation of High‐Mobility Two‐Dimensional Electron Gases at Crystalline Complex Oxide Interfaces

Well-controlled sub-unit-cell layer-bylayer epitaxial growth of spinel alumina is achieved at room temperature on a TiO2 -terminated SrTiO3 single-crystalline substrate. By tailoring the interface redox reaction, 2D electron gases with mobilities exceeding 3000 cm 2 V(-1) s(-1) are achieved at this novel oxide interface.

Molecular ordering of ethanol at the calcite surface.

To produce biominerals, such as shells, bones, and teeth, living beings create organic compounds that control the growth of the solid phase. Investigating the atomic scale behavior of individual functional groups at the mineral-fluid interface provides fundamental information that is useful for constructing accurate predictive models for natural systems. Previous investigations of the activity of coccolith-associated polysaccharides (CAP) on calcite, using atomic force microscopy (AFM) [Henriksen, K., Young, J. R., Bown, P. R., and Stipp, S. L. S. Palentology 2004, 43 (Part 3), 725-743] and molecular dynamics (MD) modeling [Yang, M., Stipp, S. L. S., and Harding, J. H. Cryst. Growth Des. 2008, 8 (11), 4066-4074], have suggested that OH functional groups control polysaccharide attachment. The purpose of this work was to characterize, using X-ray reflectivity (XR) combined with molecular dynamics (MD) simulations, the structuring on calcite of a layer of the simplest carbon chain molecule that contains an OH group, ethanol (CH(3)-CH(2)-OH). We found evidence that EtOH forms a highly ordered structure at the calcite surface, where the first layer molecules bond with calcite. The ethanol molecules stand up perpendicularly at the interface or nearly so. As a consequence, the fatty, CH(3) ends form a new surface, about 6 Å from the termination of the bulk calcite, and beyond that, there is a thin gap where ethanol density is low. Following is a more disordered layer that is two to three ethanol molecules thick, about 14 Å, where density more resembles that of bulk liquid ethanol. The good agreement between theory and experiment gives confidence that a theoretical approach can offer information about behavior in more complex systems.

Molecular Junctions Based on SAMs of Cruciform Oligo(phenylene ethynylene)s

Cruciform oligo(phenylene ethynylene)s (OPEs) with an extended tetrathiafulvalene (TTF) donor moiety (OPE5-TTF and OPE3-TTF) and their simple analogues (OPE5-S and OPE3) without conjugated substituents were used to form high-quality self-assembled monolayers (SAMs) on ultraflat gold substrates. Molecular junctions based on these SAMs were investigated using conducting-probe atomic force microscopy (CP-AFM). The TTF substituent changes the molecular orbital energy levels and decreases the HOMO-LUMO energy gap, resulting in a 9-fold increase in conductance for both TTF cruciform OPEs compared to the unsubstituted analogues. The difference in electrical transport properties of the SAMs was reproduced by the theoretical transport calculations for the single molecules.

A New Class of Extended Tetrathiafulvalene Cruciform Molecules for Molecular Electronics with Dithiafulvene‐4,5‐Dithiolate Anchoring Groups

Cruciform motifs with two orthogonally oriented π-extended tetrathiafulvalenes and with differently protected thiolate end-groups are synthesized by stepwise coupling reactions. The molecules are subjected to single-molecule conductivity studies in a break-junction and to conducting probe atomic force microscopy studies in a self-assembled monolayer on gold.