Near band-gap electronics properties and luminescence mechanisms of boron nitride nanotubes

The deep ultraviolet luminescence (h nu >= 5 eV) of multiwall boron nitride nanotubes (BNNTs) is studied with time-and energy-resolved photoluminescence spectroscopy. Two luminescence bands are observed at 5.35 and 5.54 eV. Both emissions undergo a large blue shift of several tens of meV with a linear slope Delta E-lum/Delta E-exc < 1 as the excitation energy E-exc increases. When E-exc >= 5: 8 eV, the spectral band positions become fixed, which marks the transition between the excitation of donor-acceptor pairs and creation of free charge carriers. We assign the 5.35 eV band to quasi donor-acceptor pair transitions and the band at 5.54 eV to free-bound transitions. Boron and nitrogen atoms distributed along characteristic defect lines in BNNTs should be involved in the luminescence process. The presented results permit a revision of previous assignments of electronic transitions in BNNTs. (C) 2015 AIP Publishing LLC.