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The optical properties of spherical gold and silver nanoparticles distributed in size are investigated through a new effective medium theory. It is about the modified Maxwell-Garnett-Mie theory (MMGM) that takes into account the intrinsic confinement, dynamic effects and the nanoparticle radius distribution. In the case of monodispersed nanoparticles with a radius in the 1 nm-25 nm range, MMGM is equivalent to the Mie theory. Compared to the conventional effective medium theory, we demonstrate that MGMM gives better description of the measured extinction spectra of colloidal gold and silver solutions. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The sensing performance of a micro-sensor based on a thick film of nitrogen-doped single-walled carbon nanotube (SWCNTs) is compared to a pristine SWCNTs film. Transfer length method analysis is used to extract the film sheet resistance from electrical measurements and reveals that the gas sensing mechanism is mainly attributed to the charge transfer between gas molecules and SWCNTs. We demonstrate that the sensitivity to NH3 can be improved by using a sensor based N-doped SWCNTs films while the sensitivity to NO2 and water vapour is unmodified. These unique gas-sensing properties can be used to develop a new NH3-selective differential gas sensor.

A variety of fabrication methods for single-walled carbon nanotubes (SWCNTs) has been developed, but none of them has been proven to generate tubes of only one chirality. Hence, a sample of as-produced SWCNTs always consists of a mixture of metallic and semiconducting tubes. By this, the application of SWCNTs in electronics, optics and sensing is limited today. Recently, the sorting of carbon nanotubes using density gradient ultracentrifugation (DGU) has been demonstrated. We report on an improved sorting according to electronic type by applying a two-step procedure: In the first step, we enriched semiconducting SWCNTs via DGU. In the second step, the sorted fractions have been reinserted and a second DGU was performed, winnowing metallic SWCNTs. Employing this procedure, we were able to reduce the content of metallic SWCNTs from 38 to 11%. Furthermore, we assembled carbon nanotube field-effect transistors with a high ON/OFF ratio from the sorted fraction in a single st ep by dielectrophoresis.