5 research outputs found
Mechanically-Controlled Binary Conductance Switching of a Single-Molecule Junction
Molecular-scale components are expected to be central to nanoscale electronic
devices. While molecular-scale switching has been reported in atomic quantum
point contacts, single-molecule junctions provide the additional flexibility of
tuning the on/off conductance states through molecular design. Thus far,
switching in single-molecule junctions has been attributed to changes in the
conformation or charge state of the molecule. Here, we demonstrate reversible
binary switching in a single-molecule junction by mechanical control of the
metal-molecule contact geometry. We show that 4,4'-bipyridine-gold
single-molecule junctions can be reversibly switched between two conductance
states through repeated junction elongation and compression. Using
first-principles calculations, we attribute the different measured conductance
states to distinct contact geometries at the flexible but stable N-Au bond:
conductance is low when the N-Au bond is perpendicular to the conducting
pi-system, and high otherwise. This switching mechanism, inherent to the
pyridine-gold link, could form the basis of a new class of
mechanically-activated single-molecule switches
Chapter 11 Biodiversity, biogeography and phylogeography of Ordovician rhynchonelliform brachiopods
<p>The phylogeographical evolution and the consequent changing distribution and diversity of rhynchonelliform brachiopods through
the Ordovician are linked to the dynamic palaeogeography of the period. The Early Ordovician (Tremadocian and Floian) is characterized
by globally low-diversity faunas with local biodiversity epicentres, notably on the South China Palaeoplate; low-latitude
porambonitoid-dominated faunas with early plectambonitoid and clitambonitoid representatives, as well as high-latitude assemblages
mostly dominated by orthoids, can be recognized, but many taxa are rooted in Late Cambrian stocks. The Early Ordovician displays
a steady increase in rhynchonelliformean biodiversity, which was mostly driven by the increasing success of the Porambonitoidea
and Orthoidea, but the billingsellids and early plectambonitoids also contributed to this expansion. During the Early to Mid
Ordovician (Dapingian–Darriwilian), marine life experienced an unprecedented hike in diversity at the species, genus and family
levels that firmly installed the suspension-feeding benthos as the main component of the Palaeozoic fauna. However, this may
have occurred in response to an early Darriwilian annihilation of existing clades, some of which had been most successful
during the Early Ordovician. New clades rapidly expanded. The continents were widely dispersed together with a large number
of microcontinents and volcanic arcs related to intense magmatic and tectonic activity. Climates were warm and sea-levels
were high. Pivotal to the entire diversification is the role of gamma (inter-provincial) diversity and by implication the
spread of the continents and frequency of island arcs and microcontinents. The phylogeographical analysis demonstrates that
this new palaeogeographical configuration was particularly well explored and utilized by the strophomenides, especially the
Plectambonitoidea, which radiated rapidly during this interval. The porambonitoids, on the other hand, were still in recovery
following the early Darriwilian extinctions. Orthides remained dominant, particularly at high latitudes. Biodiversity epicentres
were located on most of the larger palaeoplates, as well as within the Iapetus Ocean. Provincial patterns were disrupted during
the Sandbian and early Katian with the migration of many elements of the benthos into deeper-water regimes, enjoying a more
cosmopolitan distribution. Later Katian faunas exhibit a partition between carbonate and clastic environments. During the
latest Katian, biogeographical patterns were disrupted by polewards migrations of warm-water taxa in response to the changing
climate; possibly as a consequence of low-latitude cradles being developed in, for instance, carbonate reef settings. Many
clades were well established with especially the strophomenides beginning to outnumber the previously successful orthides,
although this process had already begun, regionally, in the mid to late Darriwilian. At the same time, atrypoid and pentameroid
clades also began to radiate in low-latitude faunas, anticipating their dominance in Silurian faunas. The Hirnantian was marked
by severe extinctions particularly across orthide-strophomenide clades within the context of few, but well-defined, climatically
controlled provincial belts.
</p
Designing molecular architecture to control diffusion and adsorption on insulating surfaces
We present the results of calculations that have been performed to simulate the adsorption and diffusion of several model molecules, consisting of two carboxylic acid binding groups connected to a molecular backbone, on the TiO2 (110) rutile surface in order to investigate the effect of molecular structure on their surface mobility. The calculations were performed using a set of interatomic potentials that have been specifically developed to correctly reproduce the molecule-surface interaction for this system, along with established potentials for the isolated surface and intramolecular interactions. These potentials were tested through a comparison of adsorption energies and diffusion barriers of prototype molecules. We show that the rigidity of the molecular structure can significantly affect both the adsorption energy and the energy barriers for diffusion on the surface. As a result of the simulations we suggest a rigid molecular structure that will maximize the diffusion barrier. Calculations such as these will enable the design of molecules in order to tailor their diffusive properties for specific applications. © 2008 American Chemical Society