C 2 O Nanotubes with Negative Strain Energies and Improvements of Thermoelectric Properties via N-Doping Predicted from First-Principle Calculations.

In this study, the geometric structure, strain energy, and electronic properties of armchair C 2 O nanotubes (A _ C 2 O N T s (n , n)) and zigzag C 2 O nanotubes   (Z _ C 2 O N T s (n , 0)) are studied in detail. The results show that these nanotubes behave as special shapes; the section of Z _ C 2 O N T s (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 _ C 2 O N T s (n , n) fluctuates. With an increase in n, the fluctuation disappears gradually, and the section also tends to be round. Compared with C 2 O nanosheets ( g _ C 2 O ), C 2 O 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 _ C 2 O N T s start to show negative strain energies from n = 8 and A _ C 2 O N T s from n = 12, indicating their higher stabilities relative to the   g _ C 2 O sheets. The calculation of the electronic band structure shows that Z _ C 2 O N T s are an indirect band gap semiconductor, and A _ C 2 O N T is a direct band gap semiconductor. The band gap value of Z _ C 2 O N T s first increases and then stabilizes with the diameters of the nanotubes, and the band gap value of A _ C 2 O N T s decreases with the increase in the nanotubes and tends to the band gap value of the g _ C 2 O sheet. In addition, the electronic properties and thermoelectric properties of C 2 O N T s (n = 4 , 6 , 8) before and after N-doping were also studied. We found that N-doping changed the electronic and thermoelectric properties of C 2 O N T s . It reduced the nanotube band gap value and significantly improved the thermoelectric figures of merit of Z _ C 2 O N T s (n = 4 , 6 , 8) and A _ C 2 O N T (4 , 4) , which comprised an effective strategy to improve the thermoelectric figure of merit of nanotubes. The results showed that the C 2 O N T s had potential as thermoelectric materials after N-doping, which provided important guidance for designing low-dimensional g _ C 2 O nanostructures. [ABSTRACT FROM AUTHOR]