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

Abstract

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 (edoping) 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 anti-bonding 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 step-like 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, 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.

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“…The RBM frequency is inversely proportional to the SWCNT’s diameter, obeying the relation ω RBM = α/ d + ω 0 , where α is the constant of proportionality between the RBM frequency and the SWCNT’s diameter and ω 0 is dependent on the SWCNT’s physicochemical environment. − Using the DFT-calculated diameters d in Å from Table , we find that α ≈ 221.8 × 10 –7 and ω 0 ≈ 17.5 cm –1 give the best fit to the RBM frequencies of ref , as shown in Figure d. Such values are consistent with those previously reported for other SWCNT samples. , …”

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“…Herein, the IG/ID values of SWNTs and SWNTs/EDC are 11.7 and 4.2, respectively, demonstrating the presence of EDC. Additionally, the diameters of the SWNTs were calculated from the radial breathing modes (RBMs) ( Kröckel et al, 2020 ). In order to analyze Raman spectra, the formula ωRBM = A/d + B is used, where A = 234 cm −1 , B = 10 cm −1 , ωRBM is the radial breathing mode, and d is the diameter of the nanotube.…”

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“…3 (b)). Previous advances have suggested that the electronical properties of quasi 1D structures such as single-walled carbon nanotubes 31 depend on their diameter and chirality. Similarly, we find that the band gap can be controlled by changing the chirality and size of the few-chain Te nanowires.…”