Formation and electronic properties of ${\text{BC}}_3$ single-wall nanotubes upon boron substitution of carbon nanotubes

We report a detailed experimental and theoretical study on the electronic and optical properties of highly boron-substituted (up to $15\text{at.}\%$) single-wall carbon nanotubes. Core-level electron energy-loss spectroscopy reveals that the boron incorporates into the lattice structure of the tubes, transferring $\sim 1∕2$ hole per boron atom into the carbon derived unoccupied density of states. The charge transfer and the calculated Fermi-energy shift in the doped nanotubes evidence that a simple rigid-band model can be ruled out and that additional effects such as charge localization and doping induced band-structure changes play an important role at this high doping levels. In optical absorption a new peak appears at $0.4\text{eV}$ which is independent of the doping level. Compared to the results from a series of ab initio calculations our results support the selective doping of semiconducting nanotubes and the formation of ${\text{BC}}_3$ nanotubes instead of a homogeneous random boron substitution.

Figure 1

$\mathrm{B}1s$ (left) and $\mathrm{C}1s$ (right) EELS spectra of boron doped SWCNT. Right inset: Expanded scale to focus on the $\mathrm{C}1s\to {\pi }^{*}$ excitation onset. The dashed lines represent reference spectra from the $\mathrm{B}1s$ excitation edge of MWBNNT (Ref. 14) and the $\mathrm{C}1s$ edge of pristine SWCNT, respectively. The spectra are normalized to the respective $1s\to {\sigma }^{*}$ excitation peaks at $199\text{eV}$ and $292\text{eV}$ for B and C, respectively.

Figure 2

Optical-absorption spectra of boron doped SWCNT (solid) and pristine SWCNT (dashed).

Figure 3

Band structure, DOS and joint DOS divided by $E^2$ of a C(16,0) tube with (a) $0\%$, (b) $6.25\%$, (c) $12.5\%$, and (d) $25\%$ boron doping. Zero energy denotes the Fermi energy. Filled states are indicated by gray shadowing. Also displayed are the corresponding unit cells (C atoms black, B atoms white).

Figure 4

(a) Band structure of ${\text{BC}}_3\left(3,3\right)$ nanotube (Fermi energy at $0\text{eV}$). (b) Density of states (filled states shadowed with gray). (c) Joint density of states divided by $E^2$ (solid line) and calculated optical absorption (dashed line) of a perfect infinite ${\text{BC}}_3\left(3,3\right)$ tube with light polarization along the tube axis. (d) Sketch of the unit cell.