Hybrid Nanostructures: New Frontiers for Technology and Raman Spectroscopy

Graphene is the two-dimensional (2d)building block for graphitic materi-als of any other dimensionality. It can be wrapped into zero-dimensional (0d) fullerenes, rolled into one-dimensional (1d) nanotubes, or stacked into three-dimen-sional (3d) graphite.1 The first evidence of nanotubes dates back to 1952,2 with signif-icant work performed in the 1970s.3 This field came of age, with an exponential increase in publications, after multiwall nano-tubes (MWNTs) were studied in 1991 with a high-resolution transmission electron microscope,4 soon followed by single-wall nanotubes (SWNTs).5 Depending on their diameter and chirality, SWNTs can be either metallic (m) or semiconducting (s). Their band gaps vary inversely with diameter.6 One of the main goals of current nanotube research is to grow on-demand tubes of well-defined chirality. However, to date, most approaches result in heterogeneous samples, with only limited success in the selective growth of s-SWNT, m-SWNT, or SWNTs with narrow chirality distributions.7 Selection can be achievedby dispersion and individualization in solution, followed by a sorting process8 such as electrophoresis, chromatography, or density gradient ultra-centrifugation, to cite a few. However, this typically requires the use of chemicals9 that may be difficult to remove andmay result in perturbations of the electronic and optical properties. Double-wall carbon nanotubes (DWNTs) are being tested in different areas,10 including electronics (e.g., field-effect transistors), photonics (e.g., ultrafast optical switches), sensors (e.g., mass sensors), and energy storage and genera-tion (e.g., supercapacitors and solar cells). The inner and outer tubes can be either metallic or semiconducting, with four dif-ferent combination