| Summary: | We have used density functional theory (DFT)
and time dependent (TD)-DFT to systematically
investigate the dependency of the geometric and vibroelectronic
properties of zigzag and armchair-type doublewalled
boron nitride nanotubes ((0,m)@(0,n) and
(m,m) @(n,n)-DWBNNTs) on the interwall distance (ΔR)
and the number of unit cells. The results of the
calculations showed that their structural stability strongly
depends on the interwall distance, but not on the number
of unit cells, and the (0,m) @(0,m+9/10) and (m,m) @(n,n)
with n=m+5/6 are the most energetically stable structures.
The predicted electronic structures for DWBNNTs with
cell lengths of one unit exhibit a strong red-shift for the
ΔR below ~0.4 nm and remain almost constant for the ΔR
> 0.45 nm. The calculated nonresonance Raman spectra of
(0,6) @(0,n)-DWBNNTs (with cell lengths of one unit and
n=12-18) indicated that the radial breathing modes
(RBMs) of inner (0,6) and outer (0,n) tubes are not only
diameter dependent, but also exhibit a strong blue-shift
for the ΔR below ~0.35 nm and rapidly approach zero
with increasing ΔR reference to the position of the RBM in
the spectrum of the corresponding single wall boron
nitride nanotubes, (0,n)-SWBNNTs. The calculated IR
spectra of the (0,6) @(0,n)-DWBNNTs did not indicate any
significant dependence on the ΔR for n > 13.
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