Ralf Thomas Weitz

Contact and edge effects in graphene devices

Electrical transport studies on graphene have been focused mainly on the linear dispersion region around the Fermi level and, in particular, on the effects associated with the quasiparticles in graphene behaving as relativistic particles known as Dirac fermions. However, some theoretical work has suggested that several features of electron transport in graphene are better described by conventional semiconductor physics. Here we use scanning photocurrent microscopy to explore the impact of electrical contacts and sheet edges on charge transport through graphene devices. The photocurrent distribution reveals the presence of potential steps that act as transport barriers at the metal contacts. Modulations in the electrical potential within the graphene sheets are also observed. Moreover, we find that the transition from the p- to n-type regime induced by electrostatic gating does not occur homogeneously within the sheets. Instead, at low carrier densities we observe the formation of p-type conducting edges surrounding a central n-type channel.

Polymer nanofibers via nozzle-free centrifugal spinning

We report on the unexpected finding of nanoscale fibers with a diameter down to 25 nm that emerge from a polymer solution during a standard spin-coating process. The fiber formation relies upon the Raleigh-Taylor instability of the spin-coated liquid film that arises due to a competition of the centrifugal force and the Laplace force induced by the surface curvature. This procedure offers an attractive alternative to electrospinning for the efficient, simple, and nozzle-free fabrication of nanoscale fibers from a variety of polymer solutions.

Strong p‐Type Doping of Individual Carbon Nanotubes by Prussian Blue Functionalization

Keywords: carbon nanotubes ; doping ; electrodeposition ; Prussian blue ; Field-Effect Transistors ; Electrochemical Preparation Method ; Modified Electrodes ; Charge-Transfer ; Single ; Oxidation ; Devices ; Bundles ; Composites ; Deposition Reference EPFL-ARTICLE-160520doi:10.1002/smll.200800803View record in Web of Science Record created on 2010-11-30, modified on 2017-05-12

Low-voltage metal-gate top-contact organic thin-film transistors and complementary inverters with submicron channel length

Since the mobility of organic semiconductors cannot be increased indefinitely , improvements in the dynamic performance of organic thin-film transistors (TFTs) require reductions in the TFT dimensions. Assuming a mobility of 0.1 cm2/Vs and aiming for a cutoff frequency of 10 MHz at 3 V, channel length and gate-to-contact overlap have to be reduced well below 1 ¿m. Although no cost-effective methods to manufacture high-mobility submicron organic TFTs on large-area flexible substrates currently exist, such methods may become available in the near future. To ensure that the carrier density in short-channel TFTs is controlled by the gate, rather than the drain, the gate dielectric thickness must also be reduced, ideally using a low-temperature-processable dielectric and without introducing large gate leakage.

Top-gate ZnO nanowire transistors with ultrathin organic gate dielectric

To take full advantage of the small size and excellent charge transport properties of inorganic nanowires, we have prepared FETs and integrated circuits based on individual single-crystalline ZnO nanowires with patterned, metallic top gate electrodes and a thin, solution-processed gate dielectric.

Integrated circuits using top-gate ZnO nanowire transistors with ultrathin organic gate dielectric

We report on field-effect transistors based on single-crystalline ZnO nanowires with a diameter of about 50 nm grown by wet-chemical synthesis. The as-grown nanowires have a large conductivity that makes it difficult to control the drain current with the gate field, but the conductivity is greatly reduced by a post-growth anneal at 600°C. Using a solution-processed organic gate dielectric with a thickness of 2.1 nm and overlapping metal top gate electrodes patterned by electron-beam lithography we have prepared nanowire transistors with good static performance. By patterning more than one transistor on the same nanowire we have also prepared simple logic circuits on a single nanowire.

Low-Voltage Organic Thin-Film Transistors with Improved Stability and Large Transconductance

Pentacene is among the most popular organic semiconductors for organic thin-film transistors (TFTs), due to its relatively large carrier mobility. However, the stability of pentacene TFTs under continuous dynamic operation may be insufficient for future applications. Here we compare the static and dynamic performance and the operational stability of low-voltage organic TFTs based on pentacene and a recently synthesized organic semiconductor, di(phenylvinyl)anthracene (DPVAnt).