Dielectric screening plays a vital role in determining physical properties at the nanoscale and affects our ability to detect and characterize nanomaterials using optical techniques. We study how dielectric screening changes electromagnetic fields and many-body effects in nanostructures encapsulated inside carbon nanotubes. First, we show that metallic outer walls reduce the scattering intensity of the inner tube by 2 orders of magnitude compared to that of air-suspended inner tubes, in line with our local field calculations. Second, we find that the dielectric shift of the optical transition energies in the inner walls is greater when the outer tube is metallic than when it is semiconducting. The magnitude of the shift suggests that the excitons in small-diameter inner metallic tubes are thermally dissociated at room temperature if the outer tube is also metallic, and in essence, we observe band-to-band transitions in thin metallic double-walled nanotubes.

A multi-step procedure was effectively employed to synthesize innovative three-dimensional (3D) heterostructures encompassing sodium titanate (Na2Ti3O7) nanowire cores, an intermediate resorcinol-formaldehyde (RF) layer, and outer silver (Ag) nanoparticle sheaths, referred to as Na2Ti3O7@RF@Ag heterostructures. Initially, a one-step hydrothermal technique facilitated the direct growth of single-crystal Na2Ti3O7 nanowires onto a flexible Ti foil. Subsequently, a two-step wet chemical process facilitated the sequential deposition of an RF layer and Ag nanoparticles onto the Na2Ti3O7 nanowires at a low reaction temperature. Optimal concentrations of silver nitrate and L-ascorbic acid can lead to the cultivation of Na2Ti3O7@RF@Ag heterostructures exhibiting heightened surface-enhanced Raman scattering (SERS), which is particularly beneficial for the detection of rhodamine B (RhB) molecules. This phenomenon can be ascribed to the distinctive geometry of the Na2Ti3O7@RF@Ag heterostructures, which offer an increased number of hot spots and surface-active sites, thereby showcasing notable SERS enhancement, commendable reproducibility, and enduring stability over the long term. Furthermore, the Na2Ti3O7@RF@Ag heterostructures demonstrate remarkable follow-up as first-order chemical kinetic and recyclable photocatalysts for the photodecomposition of an RhB solution under UV light irradiation. This result can be attributed to the enhanced inhibition of electron-hole pair recombination and increased surface-active sites.