First-principles study of a carbon nanobud

Carbon nanobuds (CNBs), a novel carbon nanostructure, have been synthesized recently via covalently bonding C-60 buckyballs to the sidewall of a single-walled carbon nanotube (SWCNT) through cycloaddition reaction [Nasibulin, A. G. et al., Nat. Nanotechnol. 2007, 2, 156]. We perform a first-principles study of structural, electronic, chemical, and field-emission properties of CNBs. It is found that relative stabilities of CNBs depend on the type of carbon-carbon bond dissociated in the cycloaddition reaction. All CNBs are semiconducting regardless of the original SWCNT base being metallic or semiconducting. Chemical attachment of C-60 to SWCNTs can either open up the band gap (e.g., for armchair SWCNT) or introduce impurity states within the band gap, thereby reducing the band gap (for semiconducting SWCNT). In addition, the band gap of CNBs can be modified by changing the density of C-60 attached to the sidewall of the SWCNT. The work function of CNBs can be either slightly higher or lower than that of the parent SWCNT, depending on whether the attached SWCNT is armchair or zigzag. Computed reaction pathway for the formation of CNBs shows that the barriers of both forward and backward reactions are quite high, confirming that CNBs are very stable at room temperature.