6. Group 13–15 Needle-Shaped Oligomers and Nanorods: Structures and Electronic Properties

Synthetic approaches, structures, and reactivity of group 13–15 needle-shaped oligomers have been reviewed. Computational studies reveal that needle-shaped oligomers are more stable than fullerene-like isomers for all 13–15 pairs. Formation of such oligomers in the gas phase is energetically favorable and feasible from the thermodynamic point of view. However, many competitive reaction pathways are kinetically possible which leads to cascade of reactions and different reaction products. A systematic study of the various effects of structural variations on the electronic properties of Ga-N-based nanorods has been performed. On the basis of DFT computations, we demonstrate that terminal groups have a crucial impact on the electronic properties of the rod-shaped [RGaNH]_3n (R=H, CH₃) oligomers. Oligomers capped with GaR and NH groups adopt almost periodic structure in which terminal groups affect only the very edges of the oligomer. The band gap energy of the [HGaNH]_3n+1 is defined by states localized at the different ends of the oligomer. The value of the band gap is converging fast with increase of n, and for n = 38 it is about 93% of the value of the band gap of the [HGaNH]_3∞ polymer. In contrast, termination of the [HGaNH]_3n rod-shaped oligomer by saturation of dangling bonds with H or CH₃ groups destroys the periodic pattern and increases the number of states, localized at the ends of the oligomer. This way of termination is characterized by systematic change in structural parameters of the oligomer and near exponential decrease of the band gap energy with the oligomer length. The band gap energy for the rod-shaped oligomer of 10 nm of length (n = 38) amounts to only 27% of the value for the band gap of the [HGaNH]_3∞ polymer. Substitution of Ga atoms by Al and In has also been considered. Absorption spectra undergo a red shift if Ga atoms are replaced by In atoms and a small blue shift if Ga atoms are replaced by Al atoms. The effect of electron-donating and electron-withdrawing terminal groups (H, CH₃, F, CF₃) on a dipole moment and energy gap values is found to be significant. The band gap energy of long tube-shaped Ga-N-based oligomers can be tuned within 2 eV by changing the substituents at the ends of the oligomer. A combined effect of all considered factors, substituent groups variations, rod’s elongation, and the way of ends’ termination, can help to vary energy gap of the [HGaNH]_3n rod-shaped oligomer within the range 1–7 eV. Potential applications and further directions are also discussed.