n a realistic threedimensional model, optical matrix element and linear optical absorption of zigzag single walled boron nitride nanotubes (BNNTs) in the tight binding approximation are studied. In terms of absolute value of dipole matrix elements of the first three direct transitions at kz=0, we divided the zigzag BNNTs into three groups and investigated their optical absorption spectrum in energy ranges E<5,7<E<7.5 and E>7.5eV. We found that in lower energies, E<5eV, all groups show different behaviors while in the higher energies, 7<E<7.5 and E>7.5eV, their behaviors depend on their even or odd nanotube index. We also found that in the energy range 7<E<7.5eV all tubes have two distinct peaks denoted by âAâ and âBâ where the âBâ peak energy position is approximately constant and is independent of the nanotube diameter. We also found that increasing the tubes diameter leads to red shift for all peaks except âAâ peak where this peak moves to higher energies. Our results are in good agreement with the experimental results.
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