Carbon Nano-onions: Potassium Intercalation and Reductive Covalent Functionalization.

Herein we report the synthesis of covalently functionalized carbon nano-onions (CNOs) via a reductive approach using unprecedented alkali-metal CNO intercalation compounds. For the first time, an in situ Raman study of the controlled intercalation process with potassium has been carried out revealing a Fano resonance in highly doped CNOs. The intercalation was further confirmed by electron energy loss spectroscopy and X-ray diffraction. Moreover, the experimental results have been rationalized with DFT calculations. Covalently functionalized CNO derivatives were synthesized by using phenyl iodide and n-hexyl iodide as electrophiles in model nucleophilic substitution reactions. The functionalized CNOs were exhaustively characterized by statistical Raman spectroscopy, thermogravimetric analysis coupled with gas chromatography and mass spectrometry, dynamic light scattering, UV-vis, and ATR-FTIR spectroscopies. This work provides important insights into the understanding of the basic principles of reductive CNOs functionalization and will pave the way for the use of CNOs in a wide range of potential applications, such as energy storage, photovoltaics, or molecular electronics.

Understanding the Electron-Doping Mechanism in Potassium-Intercalated Single-Walled Carbon Nanotubes.

Single-walled carbon nanotubes (SWCNTs) can be doped with potassium, similar to graphite, leading to intercalation compounds. These binary systems exhibit a clear metallic character. However, the entire picture of how electron doping (e-doping) modifies the SWCNTs' vibrational spectra as a function of their diameter, chirality, and metallicity is still elusive. Herein, we present a detailed study of the intercalation and solid state reduction of metallic and semiconducting enriched HiPco SWCNTs. We performed a combined experimental and theoretical study of the evolution of their Raman response with potassium exposure, focusing specifically on their radial breathing mode (RBM). We found the charge donated from the potassium atoms occupies antibonding π orbitals of the SWCNTs, weakening their C-C bonds, and reducing the RBM frequency. This RBM downshift with increasing doping level is quasi-linear with a steplike behavior when the Fermi level crosses a van Hove singularity for semiconducting species. Moreover, this weakening of the C-C bonds is greater with decreasing curvature, or increasing diameter. Overall, this suggests the RBM downshift with e-doping is proportional to both the SWCNT's integrated density of states (DOS) ϱ(ε) and diameter d. We have provided a precise and complete description of the complex electron doping mechanism in SWCNTs up to a charge density of -18 me/C, far beyond that achievable by standard gate voltage studies, not being the highest doping possible, but high enough to track the effects of doping in SWCNTs based on their excitation energy, diameter, band gap energy, chiral angle, and metallicity. This work is highly relevant to tuning the electronic properties of SWCNTs for applications in nanoelectronics, plasmonics, and thermoelectricity.