PHYSICAL REVIEW LETTERS
 Crossover from Coulomb Blockade to Quantum Hall
 Effect in Suspended Graphene Nanoribbons

 Suspended graphene nanoribbons formed during current
 annealing of suspended graphene flakes have been
 investigated experimentally. Transport measurements
 show the opening of a transport gap around charge
 neutrality due to the formation of "Coulomb islands",
 coexisting with quantum Hall conductance plateaus appearing at moderate values
 of the magnetic field B. Upon increasing B, the transport gap is rapidly suppressed,
 and is taken over by a much larger energy gap due to electronic correlations. Our
 observations show that suspended nanoribbons allow the investigation of
 phenomena that could not so far be accessed in ribbons on SiO2 substrates.


 NATURE MATERIALS
 Single-crystal organic charge-transfer interfaces
 probed using Schottky-gated heterostructures

 Organic semiconductors based on small conjugated
 molecules generally behave as insulators when undoped,
 but the heterointerfaces of two such materials can
 show electrical conductivity as large as in a metal.
 Although charge transfer is commonly invoked to explain the phenomenon, the
 details of the process and the nature of the interfacial charge carriers remain
 largely unexplored. Here we use Schottky-gated heterostructures to probe the
 conducting layer at the interface between rubrene and PDIF-CN2 single crystals.
 Gate-modulated conductivity measurements demonstrate that interfacial
 transport is due to electrons, whose mobility exhibits band-like behaviour from
 room temperature to ~150 K, and remains as high as ~1 cm2 V−1 s−1 at 30K for
 the best devices. The electron density decreases linearly with decreasing
 temperature, an observation that can be explained quantitatively on the basis of
 the heterostructure band diagram. These results elucidate the electronic structure
 of rubrene/PDIF-CN2 interfaces and show the potential of Schottky-gated organic
 heterostructures for the investigation of transport in molecular semiconductors.


 NANO LETTERS
 Quantitative Determination of the Band Gap of WS2
 with Ambipolar Ionic Liquid-Gated Transistors

 We realized ambipolar field-effect transistors by
 coupling exfoliated thin flakes of tungsten disulfide
 (WS2) with an ionic liquid dielectric. The devices show
 ideal electrical characteristics, including very steep
 subthreshold slopes for both electrons and holes and
 extremely low OFF-state currents. Thanks to these ideal characteristics, we
 determine with high precision the size of the band gap of WS2 directly from the
 gate-voltage dependence of the source-drain current. Our results demonstrate
 how a careful use of ionic liquid dielectrics offers a powerful strategy to study
 quantitatively the electronic properties of nanoscale materials.


 ADVANCED MATERIALS
 Band-Like Electron Transport in Organic Transistors
 and Implication of the Molecular Structure for
 Performance Optimization

 Understanding the microscopic processes limiting the
 charge-carrier mobility in organic field-effect transistors
 (OFETs) is as important as to improve the quality of
 existing devices.To this end, OFETs based on single
 crystals have considerable potential, owing to their unprecedented structural
 quality and chemical purity. Single-crystal transistors have been used to
 demonstrate the occurrence of band-like transport in OFETs, through observation
 of an (anisotropic) increase in carrier mobility with decreasing temperature and
 the Hall effect. They have also led to a detailed microscopic understanding of how
 transport is influenced by the gate dielectric, which has a dominant effect in
 determining the field-effect mobility that is measured experimentally. Despite
 these successes, our understanding of OFET charge transport remains limited:
 only a few materials exhibit band-like transport in an OFET configuration, in all
 cases corresponding to p-channel devices. More importantly, it is not understood
 why only those materials exhibit band-like transport. At this stage, it is crucial to
 broaden the class of organic semiconductors in which band-like transport is
 observed, and to identify mechanisms and properties – common to all of these
 materials – that favor its occurrence.


 More papers...