AC-Impedance Spectroscopic Analysis on the Charge Transport in CVD-Grown Graphene Devices with Chemically Modified Substrates.

A comprehensive study for the effect of interfacial buffer layers on the electrical transport behavior in CVD-grown graphene based devices has been performed by ac-impedance spectroscopy (IS) analysis. We examine the effects of the trap charges at graphene/SiO2 interface on the total capacitance by introducing self-assembled monolayers (SAMs). Furthermore, the charge transports in the polycrystalline graphene are characterized through the temperature-dependent IS measurement, which can be explained by the potential barrier model. The frequency-dependent conduction reveals that the conductivity of graphene is related with the mobility, which is limited by the scattering caused by charged adsorbates on SiO2 surface.

YSZ thin films with minimized grain boundary resistivity.

In recent years, interface engineering of solid electrolytes has been explored to increase their ionic conductivity and improve the performance of solid oxide fuel cells and other electrochemical power sources. It has been observed that the ionic conductivity of epitaxially grown thin films of some electrolytes is dramatically enhanced, which is often attributed to effects (e.g. strain-induced mobility changes) at the heterophase boundary with the substrate. Still largely unexplored is the possibility of manipulation of grain boundary resistivity in polycrystalline solid electrolyte films, clearly a limiting factor in their ionic conductivity. Here we report that the ionic conductivity of yttria stabilized zirconia thin films with nano-columnar grains grown on a MgO substrate nearly reaches that of the corresponding single crystal when the thickness of the films becomes less than roughly 8 nm (smaller by a factor of three at 500 °C). Using impedance spectroscopy, the grain boundary resistivity was probed as a function of film thickness. The resistivity of the grain boundaries near the film-substrate interface and film surface (within 4 nm of each) was almost entirely eliminated. This minimization of grain boundary resistivity is attributed to Mg(2+) diffusion from the MgO substrate into the YSZ grain boundaries, which is supported by time of flight secondary ion mass spectroscopy measurements. We suggest grain boundary "design" as an attractive method to obtain highly conductive solid electrolyte thin films.

Direct Determination of Field Emission across the Heterojunctions in a ZnO/Graphene Thin-Film Barristor.

Graphene barristors are a novel type of electronic switching device with excellent performance, which surpass the low on-off ratios that limit the operation of conventional graphene transistors. In barristors, a gate bias is used to vary graphene's Fermi level, which in turn controls the height and resistance of a Schottky barrier at a graphene/semiconductor heterojunction. Here we demonstrate that the switching characteristic of a thin-film ZnO/graphene device with simple geometry results from tunneling current across the Schottky barriers formed at the ZnO/graphene heterojunctions. Direct characterization of the current-voltage-temperature relationship of the heterojunctions by ac-impedance spectroscopy reveals that this relationship is controlled predominantly by field emission, unlike most graphene barristors in which thermionic emission is observed. This governing mechanism makes the device unique among graphene barristors, while also having the advantages of simple fabrication and outstanding performance.

Highly Stable Operation of Metal Oxide Nanowire Transistors in Ambient Humidity, Water, Blood, and Oxygen.

The capability for robust operation of nanoscale transistors under harsh environments is equally important as their operating parameters such as high on-currents, high mobility, and high sensing selectivity. For electronic/biomedical applications, in particular, transistor operation must be stable under diverse conditions including ambient humidity, water, blood, and oxygen. Here we demonstrate the use of a self-assembled monolayer of octadecylphosphonic acid (OD-PA) to passivate a functionalized nanowire transistor, allowing the device to operate consistently in such environments. In contrast, without passivation, the characteristics (especially the threshold voltage) of identical nanowire transistors were dramatically altered under these conditions. Furthermore, the OD-PA-passivated transistor shows no signs of long-term stability deterioration and maintains equally high sensing selectivity to light under the harsh environments because of OD-PA's optical transparency. These results demonstrate the suitability of OD-PA passivation methods for fabricating commercial nanoelectronics.