Structural and electronic properties of double-walled zigzag and armchair Zinc oxide nanotubes

In this paper, we have investigated the stability and electronic properties of double-walled ZnO nanotubes (DWZnONTs) based on density functional theory (DFT) with the SIESTA package. The calculation have been performed on the armchair (4,4)@(n,n) and (5,5)@(n,n) DWZnONTs with (n = 9 to15) and the zigzag (7,0)@(n,0) and (6,0)@(n,0) with (n = 14 to 20). The stability calculation of DWZnONTs shows that the armchair and the zigzag DWZnONTs with difference chirality of 5, (n,n)@(n + 5,n + 5) and 8, (n,0)@(n + 8,0) and inter-wall distance of about 4.6 and 4.3 angstrom are the most stable structures, respectively. Considering the electronic band structure points that all zigzag and armchair nanotubes are semiconductors having a direct bandgap. Moreover, it is revealed that the value of the bandgap increases by increasing inner and outer tube diameters and inter-wall distances, and the process of change at higher inter-wall distances be almost constant. Our results show that the inter-wall coupling diminishes the energy gap in semiconducting DWZnONTs. We found that the energy gap of DWZnONTs depends on the structure of the inner and outer walls. The consequences of this investigation can certainly be helpful in future experimental studies.