C2O Nanotubes with Negative Strain Energies and Improvements of Thermoelectric Properties via N-Doping Predicted from First-Principle Calculations

Abstract

In this study, the geometric structure, strain energy, and electronic properties of armchair C2O nanotubes (A_C2ONTs(n,n)) and zigzag C2O nanotubes  (Z_C2ONTs(n,0)) are studied in detail. The results show that these nanotubes behave as special shapes; the section of Z_C2ONTs(n,0) along the axial direction is an n-edge type, and the section tends to be round with an increase in n while the boundary of section along the axial direction for A_C2ONTs(n,n) fluctuates. With an increase in n, the fluctuation disappears gradually, and the section also tends to be round. Compared with C2O nanosheets (g_C2O), C2O nanotubes have similar or even smaller strain energy, when the tube diameter is greater than or equal to 15 Å, the strain energy begins to show a negative value, and the negative value tends to be stable as the pipe diameter increases. Z_C2ONTs start to show negative strain energies from n = 8 and A_C2ONTs from n = 12, indicating their higher stabilities relative to the g_C2O sheets. The calculation of the electronic band structure shows that Z_C2ONTs are an indirect band gap semiconductor, and A_C2ONT is a direct band gap semiconductor. The band gap value of Z_C2ONTs first increases and then stabilizes with the diameters of the nanotubes, and the band gap value of A_C2ONTs decreases with the increase in the nanotubes and tends to the band gap value of the g_C2O sheet. In addition, the electronic properties and thermoelectric properties of C2ONTs(n=4,6,8) before and after N-doping were also studied. We found that N-doping changed the electronic and thermoelectric properties of C2ONTs. It reduced the nanotube band gap value and significantly improved the thermoelectric figures of merit of Z_C2ONTs(n=4,6,8) and A_C2ONT(4,4), which comprised an effective strategy to improve the thermoelectric figure of merit of nanotubes. The results showed that the C2ONTs had potential as thermoelectric materials after N-doping, which provided important guidance for designing low-dimensional g_C2O nanostructures.