Catalyst-free synthesis and characterization of boron nitride whiskers and nanotubes

Catalyst-free transformation of h-BN in an optical furnace in the flow of nitrogen results in formation of nanotubes, whiskers and negligible quantity of melted drops on the surface of heated samples. The thread-like structures and equiaxial particles were precipitated on a silicon substrate. The phase composition of the produced material is a mixture of h-BN, two boron-enriched tetragonal phases of BN (B_51,2N and B₂₅N), tetragonal and rhombohedric phases of pure boron and amorphous phase.An approximation of the spectral dependence of optical absorption versus photon energy of an incident light was explained in terms of absorption of the corresponding phases. Tetragonal phase of B_51,2N, tetragonal and rhombohedric phases of pure boron may correspond to band gap 3.5 eV, tetragonal phase of B₂₅N - 3.8 eV and hexagonal phase of BN - 4.8 eV. This fact confirms a theoretical assumption for the effect of boron excess in BN on a band gap.