Spectroscopic characterization of single-walled carbon nanotubes carrier-doped by encapsulation of TCNQ

Tetracyano-p-quinodimethane (TCNQ) molecules encapsulated single-walled carbon nanotubes (SWNTs) were investigated by using core level and valence band x-ray photoemission spectroscopy as well as near-edge x-ray-absorption fine structure spectroscopy. The $\pi$-band and ${\pi }^{*}$-band structures in these molecular systems were investigated, and it was clearly confirmed that a charge transfer occurred in TCNQ-doped SWNTs. TCNQ molecules were negatively charged as to be expected. A depletion of the $\pi$-band density of states of the SWNTs caused by the charge transfer to the TCNQ molecules was observed.

Figure 1

(a) High-resolution XP C $1s$ spectra taken from pristine SWNT and TCNQ@SWNT samples. (b) High-resolution XP C $1s$ spectrum taken from TCNQ@SWNT after peak fit analysis. The asymmetric SWNT C $1s$ component peak was fitted with constraints (peak shape parameters) taken from the analysis of the pristine SWNT sample.

Figure 2

High-resolution XP N $1s$ spectrum taken from TCNQ@SWNT after peak fit analysis.

Figure 3

High-resolution XPS of the valence band region spectra of pristine SWNTs and the TCNQ@SWNT sample obtained with monochromatized Al $K\alpha$ radiation.

Figure 4

C $K$-edge NEXAFS of pristine SWNTs obtained at the BESSY HE SGM beamline.

Figure 5

(a) Full C $K$-edge NEXAFS of pristine SWNTs overlaid by this of the TCNQ@SWNT sample obtained at the BESSY HE SGM beamline. (b) Expanded C $K$-edge NEXAFS of pristine SWNTs and TCNQ@SWNT.

Figure 6

Expanded C $K$-edge NEXAFS of TCNQ. According to GSCF3 simulations (Ref. 20) the resonances can be attributed to excitations from the C $1s$ core level to different ${\pi }^{*}$ MO’s: $A$: cyclohexadiene carbon bonds (C4). $B$: carbene (C2) and substituted cyclohexadiene carbon (C3) bonds. $C$: cyclohexadiene carbon (C4) and cyano group $(–\mathrm{C}\mathrm{N})$ bonds (C1). $D$: cyano group $(–\mathrm{C}\mathrm{N})$ bond (C1) and substituted cyclohexadiene carbon (C3) bonds.