The electronic bandgap of a boron nitride nanotube (BNNT) can be increased for the first time using an intense ultrashort laser pulse. In contrast, the Young’s modulus, or stiffness, will decrease due to the weakening of bonds. These interesting results have been found by researchers at the University of Kassel, who have used density functional theory to investigate how the properties of a BNNT can be manipulated unconventionally.

BNNTs have promising applications because of their good biocompatibility and their superb chemical, thermal, and mechanical properties. In combination with the resistance to most acids and alkalis they are good candidates as protecting cages for encapsulated nanoparticles. These properties are in contrast to their well-known carbon counterparts, the carbon nanotubes, who are stable only to much lower temperatures and have a relatively poor biocompatibility.

Intense femtosecond-laser excitation

An intense ultrashort laser pulse is known to produce a highly excited nonthermal state in solids, where the electrons are heated up to several ten thousands of Kelvin and the ions remain initially cold. This new exotic state with hot electrons and cold ions remains constant for several 100 femtoseconds, until the electrons cool down and the ions heat up due to electron phonon coupling. In order to investigate the microscopic effects in solids after such an intense femtosecond-laser excitation, the group of Professor Dr Garcia at the University of Kassel have developed the density functional theory code CHIVES (Garcia et al. 2013).

Unconventional manipulation

Reporting in Nanotechnology, the researchers modelled the behaviour of the Young’s modulus and the electronic band gap of a (5,0) zigzag and (9,0) zigzag BNNT after an intense ultrashort laser pulse excitation. They find that the electronic band gap increases because of the strong electronic excitation. In contrast, conventional means used to manipulate solids and nanostructures, such as temperature, pressure and electronic and magnetic fields, are known to decrease the electronic bandgap of BNNTs. This unconventional nature of a femtosecond-laser excitation moving the system out of thermodynamic equilibrium, by selectively heating the electronics while leaving the ions cold, allows BNNTs to be manipulated into a new, otherwise unattainable direction.

The Young’s modulus, or measure of stiffness, was found to decrease due to the weakening of bonds, when part of the electrons is excited from bonding to antibonding orbitals by the laser.