Structural and Electronic Properties of Double-Walled Black Phosphorene Nanotubes: A Density Functional Theory Study

The exploration of new low-dimensional systems based on phosphorus atoms is an area of intensive research because of their properties. We use first-principles total energy calculations to investigate the stability and the geometrical and electronic properties of double-walled black phosphorene nanotubes (DW beta PNTs). We are dealing with a material that has not been synthesized yet. Therefore, it is important to know if it is stable, so it could be obtained experimentally. To do this, we have calculated the formation energy for several DW beta PNTs. The results show that DW beta PNTs are more stable than the SW beta PNTs. Moreover, some of the systems are even more stable than a black phosphorene monolayer. Also, it is found that for the most stable DW beta PNTs, the distance between the two concentric SW beta PNTs is similar to the distance between layers of black phosphorus. As a consequence, very small DW beta PNTs, in which this distance is not maintained, are not stable. The band gaps of DW beta PNTs are smaller than the band gaps of their respective SW beta PNTs, a trend similar to what happens in two-dimensional systems, where the band gap is reduced as the number of layer increases. In this way, it is possible to tune the band gap of a DW beta PNT, by changing the diameter of either one of the nanotubes or both. This result opens the possibility of using the nanotubes in a wide range of applications. To be able to tune the band gap is important in optoelectronic devices, such as lasers and diodes, which function in a wide range of electromagnetic regions, and for solar cells and in photocatalysis.