Geometrical and electronic properties of unstrained and strained transition metal dichalcogenide nanotubes

Abstract

We study geometries and electronic properties of unstrained and strained zigzag $\mathrm{Mo}X{}_{2}$ ($X$ = O, S, and Se) nanotubes and $\mathrm{Mo}XY$ ($X,Y$ = O, S, and Se) nanotubes in the framework of density functional theory. Using the local density approximation, we obtain fully relaxed geometries of (10,0) and (14,0) nanotubes for all $\mathrm{Mo}X{}_{2}$, and (7,0) and (28,0) nanotubes for selected $\mathrm{Mo}X{}_{2}$. From the detailed analysis of the optimized geometries it is revealed that, although in the planar case these transition metal dichalcogenides (TMDs) normally take so-called H structures, zigzag nanotubes turn into geometries between H-like and T-like nanotubes having different height for inner and outer chalcogen site. Electronic property investigations reveal that TMD nanotubes without oxygen atoms are found to have narrower gaps than TMD nanotubes with oxygen atoms. These narrow direct gaps of TMD nanotubes should be attractive properties for infrared optoelectronic devices. On the other hand, energy-gap values of the TMD nanotubes with oxygen atoms are found to depend rather strongly on the strain. Therefore, they should be potential future device materials for mechanical sensors. We also find a global strong correlation between the fundamental-gap values and some combined structural parameters in strained $\mathrm{Mo}X{}_{2}$ ($X$ = S, Se) nanotubes and planar $\mathrm{Mo}X{}_{2}$ ($X$ = S, Se), which will be useful to predict gap values of other strained and unstrained ($n,0$) nanotubes.