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Electronic structure calculations of potassium intercalated single-walled carbon nanotubes

Electronic structure calculations of potassium intercalated single-walled carbon nanotubes. Sven Stafström and Anders Hansson Department of Physics, IFM Linköping University. Introduction. Intercalation with alkali metals will transfer charge to the carbon nanotube.

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Electronic structure calculations of potassium intercalated single-walled carbon nanotubes

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  1. Electronic structure calculations of potassium intercalated single-walled carbon nanotubes Sven Stafström and Anders Hansson Department of Physics, IFM Linköping University

  2. Introduction • Intercalation with alkali metals will transfer charge to the carbon nanotube. • Raman data by Rao et al., Nature 388, 257 (1997) • EELS data by Liu et al., PRB 67, 125403 (2003) • The charge transfer results in a metallic state and enhancement of conductivity, • Lee et al ., Nature 388, 255 (1997) • Both DFT and molecular dynamics simulations show that the alkali metal atoms intercalate the hollow sites between adjacent tubes • Miyamoto et al., PRL, 74, 2993 (1995) • Gao et al. PRL, 80, 5556 (1998)

  3. Synopsis • Geometry optimizations and band structure calculations of K intercalated (4,4) and (7,0) SWCNT’s. • Ground state geometries: what is the effect of CT on the geometry. • Heat of formation: which are the most stable K concentrations/ configurations. • Electronic structure: band structure and density of states: how does CT affect the electronic structure

  4. Methodology • DFT calculations (Vienna ab initio simulation package (VASP) • Cut-off energy 400 eV • Exchange-correlation energy functional: Perdew and Wang (PW91) • Energy convergence <10 meV • Force convergence <10 meV/Å (4,4) C32K1, C32K2, C48K1, C48K2 (7,0) C28K1, C28K2, C56K1, C56K2, C84K1, C84K2

  5. Bond-lengths, pristine SWCNT’s The zigzag tubes have a more pronounced bond-length alternation pattern than the armchair tubes.

  6. Bond lengths, K intercalated systems (7,0) C28K2 (4,4) C32K2 • Charge transfer leads to occupation of orbitals with a net anti-bonding character • The zigzag tube shows a larger geometry relaxation

  7. Heats of formation • The electron affinity of the (7,0) tube is considerably larger than of the (4,4) tube. • Maximum heat of formation is obtained for the staggered phase.

  8. Band structure, K-intercalated (4,4) SWCNT

  9. Intertube interactions, (4,4) SWCNT Pristine K-intercalated, C32K2 The band-widths perpendicular to the (reciprocal) tube axis are slightly reduced upon K-intercalation.

  10. Density of states, (4,4) SWCNT The Fermi energy can enter regions of very high density of states.

  11. Band structure, K-intercalated (7,0) SWCNT The manifold of dispersive bands above –6 eV makes the (7,0) NT highly electronegative. This explains the higher Heats of formation of the K-intercalated phases as compared to the (4,4) NT.

  12. Density of states, SWCNT (7,0) (4,4)

  13. Conclusions • Narrow SWCNT’s show two different bond-lengths, the effect is particularly strong for zigzag tubes. • Upon intercalation with potassium the size of the unit cell perpendicular to the tube axis expands. • The bond-lengths are also sensitive to K-intercalation, in particular in the case of the zigzag (7,0) SWCNT. • K-intercalation results in charge transfer and shift in the Fermi energy for both the (4,4) and the (7,0) SWCNT. • The dispersion perpendicular to the tube axis is slightly reduced as a result of K-intercalation. • The Fermi energy of the (4,4) tube can be shifted to a region of very high density of states upon K-intercalation.

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