Observing the Angular Distribution of Raman Scattered Fields.

In conventional optical spectroscopy, lenses are used to focus light on the sample and to collect light scattered from the sample. Focusing increases the signal intensity, but it amounts to angular (k-space) averaging and leads to information loss. In this issue of ACS Nano, Budde and collaborators record radiation patterns of Raman scattering from a single layer of graphene, revealing the angular distribution of the scattered field. The authors show that the radiation patterns render the spatial symmetry of vibrational modes. Furthermore, their results demonstrate that depolarization effects occurring in the focal region must be taken into account for proper interpretation of Raman intensities. We outline here the working principle of this new approach and discuss future applications for studies of graphene and other low-dimensional systems.

Modulating the electronic properties along carbon nanotubes via tube-substrate interaction.

We study single wall carbon nanotubes (SWNTs) deposited on quartz. Their Raman spectrum depends on the tube-substrate morphology, and in some cases, it shows that the same SWNT-on-quartz system exhibits a mixture of semiconductor and metal behavior, depending on the orientation between the tube and the substrate. We also address the problem using electric force microscopy and ab initio calculations, both showing that the electronic properties along a single SWNT are being modulated via tube-substrate interaction.

Electron and phonon renormalization near charged defects in carbon nanotubes.

Owing to their influence on electrons and phonons, defects can significantly alter electrical conductance, and optical, mechanical and thermal properties of a material. Thus, understanding and control of defects, including dopants in low-dimensional systems, hold great promise for engineered materials and nanoscale devices. Here, we characterize experimentally the effects of a single defect on electrons and phonons in single-wall carbon nanotubes. The effects demonstrated here are unusual in that they are not caused by defect-induced symmetry breaking. Electrons and phonons are strongly coupled in sp(2) carbon systems, and a defect causes renormalization of electron and phonon energies. We find that near a negatively charged defect, the electron velocity is increased, which in turn influences lattice vibrations locally. Combining measurements on nanotube ensembles and on single nanotubes, we capture the relation between atomic response and the readily accessible macroscopic behaviour.