260
A. V. Pomogaeva and A. Y. Timoshkin
From the results, presented and discussed above, it is clear that the way of
termination of nanorods plays an important role in the controlling of their electronic
properties. Electronic properties of the open (tube-like) and closed (needle-shaped)
Ga-N-based nanorods are completely different. This holds both for the short and
long (at least up to 10 nm of length) rod-like oligomers.
All the [RGaNH] 3n+1 oligomeric rods have a charge polarization with a dipole
moment directed from the Ga-terminated end toward N-terminated end. The dipole
moment of open oligomers is much greater than for the closed ones. Since the dipole
moment is a driving force of oligomerization process [119], the growth of open rodshaped oligomers is energetically more favorable. On the other hand, the capping
reaction is energetically favorable as well. Thus, the coexistence of open and closed
oligomers of different lengths is possible from the thermodynamic point of view.
Our study of the finite [RGaNH] 3n+1 oligomers, with n ≤ 38, showed that
saturation of dangling bonds at the polarized ends of the oligomers by H atoms
or methyl groups leads to monotonic changes in structural [RGaNH] 3 units of the
oligomer. In the case of the open oligomers, Ga-N bond lengths along the main
axis monotonously decrease, and Ga-N bond lengths perpendicular to the main axis
monotonously increase from the center to the ends of the oligomer. In the case of
closed oligomers, polarization is partially compensated by capping of the rod ends
by GaR and NH groups; the structural differences are observed only for the outmost
[RGaNH] 3 rings.
Band structure of the infinite polymer obtained from computations of closed
oligomers with length ∼10 nm demonstrates that majority of MOs are delocalized.
Band structure of the [HGaNH] 3∞ polymer, extracted from the computations of
[HGaNH] 115 oligomer, exhibits indirect band gap. However, the actual band gap
energy of the finite oligomer is formed by end-localized states where HOMO is
localized at GaH-capped end and LUMO is localized at the NH-capped end. The
polarized states can provide local attractors for holes and electrons [192]. These
localized MOs reduce the energy gap of the finite closed oligomer by ∼0.46 eV
compared to the band gap of the polymer (7.0 eV). The value of the HOMOLUMO gap moderately depends on the oligomer length, and it converges to 6.54 eV
for oligomers with n ≥ 10. Oligomers with open-type termination exhibit greater
changes in electronic structure with respect to the band structure of the infinite
polymer. Multiple localized MOs of the H 3 [HGaNH] 114 H 3 oligomer have energies
within the band gap of the [HGaNH] 3∞ polymer. HOMO and LUMO of open
oligomers are strongly localized at the opposite ends of the oligomer. The HOMOLUMO gap of the open oligomers (with the exception for the smallest oligomers)
is significantly narrower than the gap of the closed oligomers with the same n. The
gap decreases dramatically with elongation of the oligomer. For oligomers with a
length of about 10 nm, the band gap energy of the open oligomer is less than 30%
of the band gap of the closed oligomer, and the band gap value (1.9 eV) is not yet
converged in case of H 3 [HGaNH] 114 H 3 oligomers.
Excited state calculations for small clusters (n = 3) predict a dominance of
HOMO → LUMO transition in the lowest excited states both for closed and open
rods. While the probability of the transition is higher for the closed oligomers, the
A. V. Pomogaeva and A. Y. Timoshkin
From the results, presented and discussed above, it is clear that the way of
termination of nanorods plays an important role in the controlling of their electronic
properties. Electronic properties of the open (tube-like) and closed (needle-shaped)
Ga-N-based nanorods are completely different. This holds both for the short and
long (at least up to 10 nm of length) rod-like oligomers.
All the [RGaNH] 3n+1 oligomeric rods have a charge polarization with a dipole
moment directed from the Ga-terminated end toward N-terminated end. The dipole
moment of open oligomers is much greater than for the closed ones. Since the dipole
moment is a driving force of oligomerization process [119], the growth of open rodshaped oligomers is energetically more favorable. On the other hand, the capping
reaction is energetically favorable as well. Thus, the coexistence of open and closed
oligomers of different lengths is possible from the thermodynamic point of view.
Our study of the finite [RGaNH] 3n+1 oligomers, with n ≤ 38, showed that
saturation of dangling bonds at the polarized ends of the oligomers by H atoms
or methyl groups leads to monotonic changes in structural [RGaNH] 3 units of the
oligomer. In the case of the open oligomers, Ga-N bond lengths along the main
axis monotonously decrease, and Ga-N bond lengths perpendicular to the main axis
monotonously increase from the center to the ends of the oligomer. In the case of
closed oligomers, polarization is partially compensated by capping of the rod ends
by GaR and NH groups; the structural differences are observed only for the outmost
[RGaNH] 3 rings.
Band structure of the infinite polymer obtained from computations of closed
oligomers with length ∼10 nm demonstrates that majority of MOs are delocalized.
Band structure of the [HGaNH] 3∞ polymer, extracted from the computations of
[HGaNH] 115 oligomer, exhibits indirect band gap. However, the actual band gap
energy of the finite oligomer is formed by end-localized states where HOMO is
localized at GaH-capped end and LUMO is localized at the NH-capped end. The
polarized states can provide local attractors for holes and electrons [192]. These
localized MOs reduce the energy gap of the finite closed oligomer by ∼0.46 eV
compared to the band gap of the polymer (7.0 eV). The value of the HOMOLUMO gap moderately depends on the oligomer length, and it converges to 6.54 eV
for oligomers with n ≥ 10. Oligomers with open-type termination exhibit greater
changes in electronic structure with respect to the band structure of the infinite
polymer. Multiple localized MOs of the H 3 [HGaNH] 114 H 3 oligomer have energies
within the band gap of the [HGaNH] 3∞ polymer. HOMO and LUMO of open
oligomers are strongly localized at the opposite ends of the oligomer. The HOMOLUMO gap of the open oligomers (with the exception for the smallest oligomers)
is significantly narrower than the gap of the closed oligomers with the same n. The
gap decreases dramatically with elongation of the oligomer. For oligomers with a
length of about 10 nm, the band gap energy of the open oligomer is less than 30%
of the band gap of the closed oligomer, and the band gap value (1.9 eV) is not yet
converged in case of H 3 [HGaNH] 114 H 3 oligomers.
Excited state calculations for small clusters (n = 3) predict a dominance of
HOMO → LUMO transition in the lowest excited states both for closed and open
rods. While the probability of the transition is higher for the closed oligomers, the