Voltage-dependent conductance of a single graphene nanoribbon

Graphene nanoribbons could potentially be used to create molecular wires with tailored conductance properties. However, understanding charge transport through a single molecule requires length-dependent conductance measurements and a systematic variation of the electrode potentials relative to the electronic states of the molecule1, 2. Here, we show that the conductance properties of a single molecule can be correlated with its electronic states. Using a scanning tunnelling microscope, the electronic structure of a long and narrow graphene nanoribbon, which is adsorbed on a Au(111) surface, is spatially mapped and its conductance then measured by lifting the molecule off the surface with the tip of the microscope. The tunnelling decay length is measured over a wide range of bias voltages, from the localized Tamm states over the gap up to the delocalized occupied and unoccupied electronic states of the nanoribbon. We also show how the conductance depends on the precise atomic structure and bending of the molecule in the junction, illustrating the importance of the edge states and a planar geometry

Conductance of a single conjugated polymer as a continuous function of its length

The development of electronic devices at the single-molecule scale requires detailed understanding of charge transport through individual molecular wires. To characterize the electrical conductance, it is necessary to vary the length of a single molecular wire, contacted to two electrodes, in a controlled way. Such studies usually determine the conductance of a certain molecular species with one specific length. We measure the conductance and mechanical characteristics of a single polyfluorene wire by pulling it up from a Au(111) surface with the tip of a scanning tunneling microscope, thus continuously changing its length up to more than 20 nanometers. The conductance curves show not only an exponential decay but also characteristic oscillations as one molecular unit after another is detached from the surface during stretching

Single Molecular Wires Connecting Metallic and Insulating Surface Areas

Totally wired: A particular molecular adsorption geometry can be prepared by adsorbing single conjugated polyfluorene chains partially on a clean Au(111) surface and partially on a thin crystalline NaCl film, thus connecting metallic and insulating surface areas. This configuration allows the electronic characterization of one and the same molecular wire as a function of its atomic-scale environment in a planar configuration

Einzelne molekulare Drähte verbinden metallische und isolierende Oberflächenbereiche

Verdrahtet: Eine spezielle molekulare Adsorptionskonfiguration lässt sich präparieren, indem einzelne konjugierte Polyfluorenketten teils auf einer Au(111)-Oberfläche und teils auf einem dünnen kristallinen NaCl-Film adsorbieren und so die metallischen und isolierenden Oberflächenbereiche verbinden. Diese Konfiguration ermöglicht die elektronische Charakterisierung eines Drahtes als Funktion seiner atomaren Umgebung in planarer Konfiguration

Mechanical behavior of nanocrystalline NaCl islands on Cu(111)

The mechanical response of ultrathin NaCl crystallites of nanometer dimensions upon manipulation with the tip of a scanning tunneling microscope (STM) is investigated, expanding STM manipulation to various nanostructuring modes of inorganic materials as cutting, moving, and cracking. In the light of theoretical calculations, our results reveal that atomic-scale NaCl islands can behave elastically and follow a classical Hooke’s law. When the elastic limit of the nanocrystallites is reached, the STM tip induces atomic dislocations and consequently the regime of plastic deformation is entered. Our methodology is paving the way to understand the mechanical behavior and properties of other nanoscale materials

Climate-driven changes in lake conditions during MIS 3 and 2: a high resolution geochemical record from Les Echets, France

International audienceIce-core (Dansgaard et al. 1993; Johnsen et al. 2001; NGRIP members 2004) and marine sediment records (Bond et al. 1992; Moreno et al. 2004; Rasmussen & Thomsen 2004) spanning the last glacial cycle provide compelling evidence of multiple reorganizations of the climatic system triggered by changes thought to originate in the North Atlantic region (Broecker et al. 1992; Clark et al. 2002). Sudden shifts in air temperature from a cool climate to interstadial values, known as Dansgaard-Oeschger events (DO), have been active most notably during Marine Isotope Stage (MIS) 3. Abrupt and large in amplitude, DO cycles operated on a millennial to centennial time scale and are best expressed in the North Atlantic region (Dansgaard et al. 1993; Allen et al. 1999; NGRIP members 2004; Moreno et al. 2005; Grimm et al. 2006; Wohlfarth et al. 2008), though recent research suggests that these events were probably important on a global scale (e.g. Voelker et al. 2002). Iceberg surges, known as Heinrich events (H events), appear in marine records as sudden cold spells associated with a drastic reduction in sea surface temperature, a southern shift of the Polar Front, disruption of North Atlantic thermohaline circulation and substantial delivery of ice-borne detritus to the open ocean, reaching as far south as Portugal (Bond et al. 1992; Broecker et al. 1992; Bard et al. 2000; Hemming 2004). On the European mainland, lake sediments are the most promising archives for recording long-term and short-term climatic changes (Voelker et al. 2002). The few long lacustrine records from continental Europe show that, on a broad scale, long-term palaeoclimate variations are expressed clearly through changes in vegetation composition and dynamics (Woillard & Mook 1982; de Beaulieu & Reille 1984; Guiot et al. 1989; Allen & Huntley 2000; de Beaulieu et al. 2001; Guiter et al. 2003). However, in the context of abrupt climate change (e.g. for most of MIS 3), only limited palaeoecological information is available: most of the data come from sites located on the southern peninsulas of Mediterranean Europe (Voelker et al. 2002). It has been suggested that changes in plant cover varied in this region in concert with climatic fluctuations recorded in Greenland ice cores, with a diverse range of thermophilous taxa present at any time (Allen et al. 1999; Sánchez Goñi et al. 2002; Tzedakis et al. 2004). Much less floristic variability is seen, however, during MIS 3-2 in pollen records from sites located north of the main mountain ranges of central and northern Europe. Here, data from peat deposits (Behre 1989; Preusser 2004), lacustrine sediments (Woillard & Mook 1982; de Beaulieu & Reille 1984; Helmens et al. 2000; Müller et al. 2003; Sirocko et al. 2005; Engels et al. 2008) and terrestrial proxies in marine cores (Sánchez Goñi et al. 2008) reveal long cold intervals interrupted sporadically by weakly expressed interstadials marked by rises in boreal tree pollen (Allen & Huntley 2000; Guiter et al. 2003). Such significant differences in ecological responses point to strong regional and local climatic gradients associated with these events, issues that are not sufficiently considered when inferring synchronicity of events between various records. In order to improve the understanding of how the European mainland was affected by rapid climate changes, we show that important palaeoecological information can be extracted from geochemical lake records (e.g. Lallier-Vergès et al. 1993; Bernasconi et al. 1997; Meyers 1997; Dean 1999; Meyers & Lallier-Vergès 1999; Meyers & Teranes 2001; Talbot 2001; Leng et al. 2005). Les Echets (Fig. 1A, B) is situated in a region sensitive to current climate changes at the boundary between humid Atlantic air masses and Mediterranean influences. The physical and chemical properties of the former lake and catchment area underwent significant changes on millennial to centennial time scales, changes that we attempt to quantify in palaeoenvironmental terms through multi-proxy geochemical analyses of the organic fraction within sediments. We demonstrate that the region was greatly influenced by the rapid climate variability specific to MIS 3, as seen in marine sediments (Bond et al. 1992; Moreno et al. 2004; Rasmussen & Thomsen 2004; Sánchez Goñi et al. 2008) or ice cores (Johnsen et al. 2001; NGRIP members 2004) hundreds of kilometres away

The mathematics of a quantum Hamiltonian computing half adder Boolean logic gate

cited By 3International audienceThe mathematics behind the quantum Hamiltonian computing (QHC) approach of designing Boolean logic gates with a quantum system are given. Using the quantum eigenvalue repulsion effect, the QHC AND, NAND, OR, NOR, XOR, and NXOR Hamiltonian Boolean matrices are constructed. This is applied to the construction of a QHC half adder Hamiltonian matrix requiring only six quantum states to fullfil a half Boolean logical truth table. The QHC design rules open a nano-architectronic way of constructing Boolean logic gates inside a single molecule or atom by atom at the surface of a passivated semi-conductor