Recently, individual single-walled carbon nanotubes (SWNTs) functionalized with azo-benzene chromophores were shown to form a new class of hybrid nanomaterials for optoelectronics applications. Here we use a number of experimental techniques and theo
ry to understand the binding, orientation, and nature of coupling between chromophores and the nanotubes, all of which are of relevance to future optimization of these hybrid materials. We find that the binding energy between chromophores and nanotubes depends strongly on the type of tether that is used to bind the chromophores to the nanotubes, with pyrene tethers resulting in more than 90% of the bound chromophores during processing. DFT calculations show that the binding energy of the chromophores to the nanotubes is maximized for chromophores parallel to the nanotube sidewall, even with the use of tethers; second harmonic generation shows that there is nonetheless a partial radial orientation of the chromophores on the nanotubes. We find weak electronic coupling between the chromophores and the SWNTs, consistent with non-covalent binding. The chromophore-nanotube coupling, while weak, is sufficient to quench the chromophore fluorescence. Stern-Volmer plots are non-linear, which supports a combination of static and dynamic quenching processes. The chromophore orientation is an important variable for chromophore-nanotube phototransistors, and our experiments suggest the possibility for further optimizing this orientational degree of freedom.
The electronic properties of heterojunction electron gases formed in GaN/AlGaN core/shell nanowires with hexagonal and triangular cross-sections are studied theoretically. We show that at nanoscale dimensions, the non-polar hexagonal system exhibits
degenerate quasi-one-dimensional electron gases at the hexagon corners, which transition to a core-centered electron gas at lower doping. In contrast, polar triangular core/shell nanowires show either a non-degenerate electron gas on the polar face or a single quasi-one-dimensional electron gas at the corner opposite the polar face, depending on the termination of the polar face. More generally, our results indicate that electron gases in closed nanoscale systems are qualitatively different from their bulk counterparts.
