The importance of defects for carbon nanoribbon based electronics

The utilization of graphene nanoribbons for next generation nanoelectronics is commonly expected to depend on the controlled synthesis that yields a low density of defects. Edge roughness and vacancies have been shown to have a large impact on the performance of graphene nanoribbon transistors. In contrast, we show how certain defects can be used to enhance the electronic and magnetic properties of graphene nanoribbons. We explore the properties of hybrid graphene nanoribbons with armchair and zigzag features joined by an array of pentagon–heptagon structural defects. The graphene nanoribbons display an increased density of states at the Fermi level, and half metallicity in absence of external fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of Schottky-Barrier carbon nanotube field-effect transistor by an efficient semi-classical numerical modeling

An efficient semi-classical numerical modeling approach has been developed to simulate the coaxial Schottky-barrier carbon nanotube field-effect transistor (SB-CNTFET). In the modeling, the electrostatic potential of the CNT is obtained by self-consistently solving the analytic expression of CNT carrier distribution and the cylindrical Poisson equation, which significantly enhances the computational efficiency and simultaneously present a result in good agreement to that obtained from the non-equilibrium Green's function (NEGF) formalism based on the first principle. With this method, the effects of the CNT diameter, power supply voltage, thickness and dielectric constant of gate insulator on the device performance are investigated.     

Epitaxial graphene: the material for graphene electronics

The search for an ideal graphene sheet has been a quest driving graphene research. While most research has focused on exfoliated graphene, intrinsic substrate interactions and mechanical disorder have precluded the observation of a number of graphene's expected physical properties in this material. The only graphene candidate that has demonstrated all the essential properties of an ideal sheet is multilayer graphene grown on the SiC(000 ) surface. Its unique stacking allows nearly all the sheets in the stack to behave like isolated graphene, while the weak graphene‐graphene interaction prevents any significant doping or distortion in the band near the Fermi level. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Piezoresistivity in GaAs/InxGa1–xAs/AlAs superlattice structures

We report on a strong piezoresistive effect in GaAs/ In_x Ga_1–x As/AlAs superlattice structures fabricated on a GaAs‐base cantilever. The measurements of the piezoresistive properties were performed for tensile strains by static pressure experiments. The maximum gauge factor (GF) for the GaAs/In_x Ga_1–x As/AlAs epilayer can be estimated to 200, which is higher than the value of the gauge factor reported for Si transducers. Our results demonstrate a higher potential of GaAs/In_x Ga_1–x As/AlAs superlattice structures for the development of piezoresistive sensors. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chemical approach to the realization of electronic devices in epitaxial graphene

We have shown that nitrophenyl groups may be added to the surface of few‐layer epitaxial graphene (EG) by the formation of covalent carbon–carbon bonds thereby changing the electronic structure and transport properties of EG from near‐metallic to semiconducting. In the present Letter we discuss the opportunities afforded by such chemical processes to engineer device functionality in graphene by modification of the electronic properties without physical patterning.

     

Electrical properties of transparent carbon nanotube networks prepared through different techniques

Ultra‐thin, optically transparent and electrically conducting films of pure carbon nanotubes (CNTs) are widely studied thanks to their promise for broad applications. In the present work, we study and compare different deposition techniques for the production of these networks: dip‐coating, spray‐coating, vacuum filtration and electrophoretic deposition on a quartz glass using single‐walled carbon nanotubes (SWCNTs) produced by the HiPCo method. In order to optimize the networks, besides the various deposition techniques we also investigate how the optical and electrical properties vary if the networks are fabricated from different CNTs, all synthesized by the CVD method: SWCNTs, DWCNTs and MWCNTs. As the main criteria for evaluating the quality of these CNT networks we measure the electrical surface resistance at a certain optical transmittance and correlate it to the morphology (homogeneity and roughness) of the networks. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First‐principle investigation of strain effects on the electronic properties of germanium nanowires

Using the GGA functional in density functional theory, the effects of axial strain on the band structure and effective mass of narrow [110] and [100] germanium nanowires are investigated. It is observed that both compressive and tensile strain cause indirect‐to‐direct bandgap transitions. One percent of tensile strain can cause a 40 meV change in the bandgap of [110] nanowires. Effective masses of electrons and holes are subject to a change of 3–4 times in the strain‐induced transition point. This change translates into a density of state modulation which opens new possibilities for the construction of Ge nanowire‐based sensors. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman study of photo‐induced degradation of the Ru(II) complex adsorbed on nanocrystalline TiO2 films

Photo‐induced degradation of a monolayer of the Ru(II) complex adsorbed on anatase TiO₂ thin films was studied by using resonant micro‐Raman spectroscopy. We developed two contrastive experiments to analyze the degradation mechanism. An exponential decay law was found when the dye was irradiated in the absence of a reducing agent. While the sensitized TiO₂ thin film electrode was covered by the I^–/I₃^– redox couple, the dye degradation exhibited a slowed linear decay. The experimental result was compared and the degradation mechanism was analyzed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Pentagonal ZnO nanorods

We report the growth of ZnO nanorods with pentagonal cross‐section in electrodeposition on p‐type Si substrates. Cathodic potentials smaller than those used for growth of hexagonal nanorods are used. The pentagonal wires have a typical length of 1–2 µm and grow out of an inhomogeneous nano‐crystalline thin film. We tentatively explain the occurrence of the pentagonal morphology in terms of kinetic limitations for electron transport in the p‐Si substrate and changed chemical conditions at the growth surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Carbon nanotube transistors with graphene oxide films as gate dielectrics

Carbon nanomaterials,including the one-dimensional(1-D) carbon nanotube(CNT) and two-dimensional(2-D) graphene,are heralded as ideal candidates for next generation nanoelectronics.An essential component for the development of advanced nanoelectronics devices is processing-compatible oxide.Here,in analogy to the widespread use of silicon dioxide(SiO2) in silicon microelectronic industry,we report the proof-of-principle use of graphite oxide(GO) as a gate dielectrics for CNT field-effect transistor(FET) via a...     

Hot filament effects on CVD of carbon nanotubes

We grew vertically aligned CNTs via HFCVD using mixtures of methane and hydrogen as feedstock, and investigated the dependence of CNT growth on feedstock composition, filament temperature, and filament types. At the filament temperature of 2050 °C tungsten filaments were more efficient for CNT growth than tantalum ones, and higher CNT growth rates were observed when tungsten filaments were operated at 1900 °C. Regardless of filament temperatures and types, monotonic increase in growth rate of vertically aligned CNTs was observed as we increased the methane concentration in the feedstock. In‐situ investigation of feedstock dissociation revealed the generation of various radical species, and, moreover, a strong correlation between CNT growth rates and relative mole fractions of single‐carbon radicals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanoscale resistive switching in SrTiO3 thin films

The local conductivity of SrTiO₃ thin films epitaxially grown on SrRuO₃‐buffered SrTiO₃ single crystals has been investigated in detail with an atomic force microscope equipped with a conducting tip (LC‐AFM). These experiments demonstrate that the conductivity of SrTiO₃ thin films originates from nanoscale well‐conducting filaments connecting the surface to the SrRuO₃ bottom electrode. The electrical conduction of the filaments is shown to be reversibly modulated over several orders of magnitude by application of an appropriate electrical field. We analyze the resistive switching by addressing individual filaments with the AFM tip as well as by scanning areas up to the µm scale. Temperature dependent measurements reveal that resistive switching on a macroscopic scale can be traced down to the insulator‐to‐metal transition of the independently switchable filaments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two-dimensional topological insulator state and topological phase transition in bilayer graphene

We show that gated bilayer graphene hosts a strong topological insulator (TI) phase in the presence of Rashba spin-orbit (SO) coupling. We find that gated bilayer graphene under preserved time-reversal symmetry is a quantum valley Hall insulator for small Rashba SO coupling λ(R), and transitions to a strong TI when λ(R)>√[U(2)+t(⊥)(2)], where U and t(⊥) are, respectively, the interlayer potential and tunneling energy. Different from a conventional quantum spin Hall state, the edge modes of our strong TI phase exhibit both spin and valley filtering, and thus share the properties of both quantum spin Hall and quantum valley Hall insulators. The strong TI phase remains robust in the presence of weak graphene intrinsic SO coupling.     

Magnetoelectric oxide films for spin manipulation in graphene

The challenge of creating a graphene spin field effect transistor (spin‐FET) demands a magnetic gate dielectric material whose magnetization can be switched electrically. We have grown films of Cr₂O₃ on top of graphite and graphene by pulsed laser deposition that shows this crucial functionality. We demonstrate that the Cr₂O₃ films are magnetoelectric by poling them in combined electric and magnetic fields and then using magnetic force microscopy to observe spontaneous surface domain structure as a function of poling field. In addition, we show that the electric field created by a conducting AFM tip can be used to write magnetic patterns in the film that demonstrate the kind of continuous magnetoelectric control needed for a prototype spin‐FET. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Band bending and quasi‐2DEG in the metallized β‐SiC(001) surface

We study the mechanism leading to the metallization of the β‐SiC(001) Si‐rich surface induced by hydrogen adsorption. We analyze the effects of band bending and demonstrate the existence of a quasi‐2D electron gas, which originates from the donation of electrons from adsorbed hydrogen to bulk conduction states. We also provide a simple model that captures the main features of the results of first‐principles calculations, and uncovers the basic physics of the process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nanowire manipulation on surfaces through electrostatic self‐assembly and magnetic interactions

A method for fabricating aligned nanowire arrays on surfaces is shown. Gold and segmented Au/Ni/Au nanowires of high aspect ratio have been prepared by template electrosynthesis, and functionalized with charged short alkanethiols that can be ionized in aqueous solutions. Different distributions of funtionalized nanowires could be obtained on large surfaces from nanowire aqueous suspensions, avoiding aggregation due to electrical repulsion. Due to the high magnetic anisotropy of segmented Au/Ni/Au nanowires chaining of aligned nanowires could be obtained by applying a low magnetic field. While electrostatics favours side wire interactions due to the high aspect ratio, concurrent electrostatics and applied magnetic field yields end‐to‐end interaction and linear alignment without bifunctionalization. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitonic absorption spectra and ultrafast dephasing dynamics in arbitrary carbon nanotubes

We illustrate the potential of the density matrix theory for investigation of optical properties of arbitrary single‐walled carbon nanotubes (CNTs). We have performed microscopic calculations of excitonic absorption spectra for CNTs of different chiral angles and diameters. The obtained results are in good agreement with experiments, in particular the excitonic binding energies match well both experiments and ab initio calculations. Furthermore, we show the strength of our approach by presenting calculations of the ultrafast Coulomb driven non‐equilibrium dynamics in CNTs. We find excitation induced dephasing on the picosecond time scale depending on the excitation strength. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Doped single‐walled carbon nanotubes and low‐mobility graphene: impact of disorder and dopants on electronic magnetotransport

The impact of disorder introduced by intentional and unintentional (environmental) dopants on the charge transport has been investigated in boron‐ and nitrogen‐doped single‐walled carbon nanotubes, and in low‐mobility monolayer graphene. For doped single‐walled nanotubes a theoretically not predicted plateau‐like region and onsets of oscillatory behaviour in the conductance for boron‐ and nitrogen‐doped nanotubes were observed, respectively. The oscillatory behaviour suggests interplay between the dopants and the helical movement of charges along the tube due to the magnetic field. For low‐mobility graphene the simultaneous appearance of the filling‐factor 2 and 3 plateau was observed in the Quantum‐Hall regime. This is attributed to an electron‐spin–isospin interaction mediated through the high disorder. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

How the SiC substrate impacts graphene's atomic and electronic structure

Graphene, the two‐dimensional form of carbon presents outstanding electronic and transport properties. This gives hope for the development of applications in nanoelectronics. However, for industrial purpose, graphene has to be supported by a substrate. We focus here on the graphene‐on‐SiC system to discuss how the SiC substrate interacts with the graphene layer and to show the effect of the interface on graphene atomic and electronic structures.

     

Metal-oxide-semiconductor field effect transistor using ‘oxidizedμc-Si/ultrathin oxide’ gate structure

The metal-oxide-semiconductor (MOS) field effect transistor (FET) using ‘oxidized μ c-Si/ultrathin oxide’ gate structure was studied. It was found that this structure shows negative differential resistance behavior, which can be explained by the Coulomb blockade effect of trapped carriers and immediate tunneling into and tunneling out with gate bias variation. The requirements for the device with this structure showing negative differential resistance behavior are based on very weak resistive coupling between floating gate and channel. They are the thinness of the tunnel oxide film, the thickness ratio of the upper oxidized film and the tunnel oxide, and the channel threshold voltage. MOSFET with this gate structure is proposed as a new negative differential resistance device.