Researchers at the National Institute for Materials Science (NIMS) in Tsukuba, Japan, have a long history of studying BN nanotubes (BNNTs), which are the structural analogs of carbon nanotubes (CNTs). While a bulk hexagonal BN single crystal can exhibit an intense near band-edge emission (NBEE), the room temperature NBEE in BNNTs is always very weak. To find out the reasons behind this weakness, the group prepared BN tubes with diameters on the microscale.

To obtain BNMTs with uniform diameters and ultra-thin walls, the researchers used Li2CO3 and boron as reactants and successfully synthesized BNMTs in a vertical induction furnace. The microtubes had diameters of 1–3 µm and well structured ultra-thin walls measuring just 50 nm in thickness. The formation of the thin-walled BNMTs can be explained by a mechanism that combines vapor-liquid-solid and template self-sacrificing processes, in which a Li2O-B2O3 eutectic reaction plays an important role.

Cathodoluminescence studies show that even at room temperature the thin-walled BN microtubes possess an intense NBEE at ~216.5 nm. Analysis using a high-resolution transmission electron microscope highlights the lack of surface curvature compared with BNNTs, which makes the walls of BNMTs appear very similar to bulk h-BN from a structural point of view. The strain energy, which induces defects, was thus significantly reduced. Therefore, the NBEE in BNMTs is dominated by the free-exciton recombinations, as is found for bulk pure BN single crystals. This is distinct compared with BNNTs, whose NBEE is attributed to the bound excitonic recombinations and, as a rule, vanishes at room temperature.

BNMTs as new one-dimensional hollow structures with a spacious (micrometer-sized) internal channel and the capability for intense deep-UV emission may find a wide range of applications in compact laser devices, microreactors and microfluidic systems.