Electronic transport in graphitic nanoribbon films.

We fabricate, pattern, and analyze thin films composed of multilayer graphene nanoribbons. These films are conductive at room temperature but depict noticeable insulating behavior at low temperatures (<20 K) due to their disordered structure. We study the transport in this strong localization regime by analyzing the dependence of resistivity on temperature and electric and magnetic fields in the framework of the variable range hopping theory. Resistivity dependence on the magnetic field confirms the insulating behavior of the films and can be fitted effectively by forward interference scattering and wave function shrinkage models at low and high magnetic field regimes, respectively. We extract large localization lengths in the range of ∼45-90 nm from both the magnetic and electric field dependence of resistivity and relate these values to the high conductance in the nanoribbons and/or good contact between them. By revealing the fundamental structural and transport properties of graphitic nanoribbon films, our results help devise methods to further improve these films for electronic and photonic device applications.

Patterned growth of silicon oxide nanowires from iron ion implanted SiO2 substrates.

We demonstrate experimentally a simple and efficient approach for silicon oxide nanowire growth, by implanting Fe(+) ions into thermally grown SiO(2) layers on Si wafers and subsequently annealing in argon and hydrogen to nucleate the nanowires. We study the effect of implantation dose and energy, growth temperature, H(2) gas flow, and growth time on the silicon oxide nanowire growth. We find that sufficiently high implant dose, high growth temperature, and the presence of H(2) gas flow are crucial parameters for silicon oxide nanowire growth. We also demonstrate the patterned growth of silicon oxide nanowires in localized areas by lithographic patterning and etching of the implanted SiO(2) substrates before growth. We propose a simple physical model to explain the growth results. This works opens up the possibility of growing silicon oxide nanowires directly from solid substrates, controlling the location of nanowires at the submicron scale, and integrating them into nonplanar three-dimensional nanoscale device structures.

Resistivity in percolation networks of one-dimensional elements with a length distribution.

One-dimensional (1D) nanoelements, such as nanotubes and nanowires, making up percolation networks are typically modeled as fixed length sticks in order to calculate their electrical properties. In reality, however, the lengths of these 1D nanoelements comprising such networks are not constant, rather they exhibit a length distribution. Using Monte Carlo simulations, we have studied the effect of this nanotube and/or nanowire length distribution on the resistivity in 1D nanoelement percolation networks. We find that, for junction resistance-dominated random networks, the resistivity correlates with root-mean-square element length, whereas for element resistance-dominated random networks, the resistivity scales with average element length. If the elements are preferentially aligned, we find that these two trends shift toward higher power means. We explain the physical origins of these simulation results using geometrical arguments. These results emphasize the importance of the element length distribution in determining the resistivity in these networks.

Micromachined silicon transmission electron microscopy grids for direct characterization of as-grown nanotubes.

Transmission electron microscopy (TEM) is a key technique in the structural characterization of carbon nanotubes. For device applications, carbon nanotubes are typically grown by chemical vapour deposition (CVD) on silicon substrates. However, TEM requires very thin samples, which are electron transparent. Therefore, for TEM analysis, CVD grown nanotubes are typically deposited on commercial TEM grids by post-processing. However, this procedure can damage the nanotubes, and it does not work reliably if the nanotube density is too low. The ability to do TEM directly on as-grown nanotubes on the silicon substrate would solve these problems. For this purpose, we have fabricated micromachined silicon TEM grids with narrow open slits on them. Since the nanotubes grown on these substrates are suspended freely over the open slits, the micromachined substrates form a natural TEM grid for direct imaging of CVD grown nanotubes. Furthermore, the background noise is significantly reduced during micro-Raman spectroscopy, resulting in a better signal-to-noise ratio. As a result, these micromachined Si substrates provide a low cost, mass producible, efficient, and reliable platform for direct TEM, SEM, AFM, and Raman characterization of as-grown nanotubes. These grids can be used for characterizing a wide range of other nanomaterials, including peapods, nanowires, and nanofibres.

High-kappa dielectrics for advanced carbon-nanotube transistors and logic gates.

The integration of materials having a high dielectric constant (high-kappa) into carbon-nanotube transistors promises to push the performance limit for molecular electronics. Here, high-kappa (approximately 25) zirconium oxide thin-films (approximately 8 nm) are formed on top of individual single-walled carbon nanotubes by atomic-layer deposition and used as gate dielectrics for nanotube field-effect transistors. The p-type transistors exhibit subthreshold swings of S approximately 70 mV per decade, approaching the room-temperature theoretical limit for field-effect transistors. Key transistor performance parameters, transconductance and carrier mobility reach 6,000 S x m(-1) (12 microS per tube) and 3,000 cm2 x V(-1) x s(-1) respectively. N-type field-effect transistors obtained by annealing the devices in hydrogen exhibit S approximately 90 mV per decade. High voltage gains of up to 60 are obtained for complementary nanotube-based inverters. The atomic-layer deposition process affords gate insulators with high capacitance while being chemically benign to nanotubes, a key to the integration of advanced dielectrics into molecular electronics.