DFT calculation of square MoS2 nanotubes

https://doi.org/10.1016/j.physe.2021.114693Get rights and content

Highlights

  • Stability and electronic structures of MoS2 nanotubes with square cross sections, which was actually synthesized in CVD experiments.

  • Five-coordinated Mo exists at the corners.

  • 25A is the threshold diameter of stable square-shaped nanotubes.

  • A singularity was observed in the effective mass of the carriers in the zigzag chirality, which correspond to the localization of the corner states.

Abstract

This paper present computational study on molybdenum disulfide (MoS2) nanotubes with square cross sections in order to elucidate the growth mechanism and properties of experimentally synthesized nanotubes. The results show that these square nanotubes have strain energies lower than the traditional cylinder ones in the small-diameter region. They also show a much smaller band gap and higher surface energies compared with the cylindrical ones. Zigzag and armchair chirality shows qualitatively different electronic structures about the localization of the edge states.

Introduction

The research on nanotube started with the discovery of variety helicities in carbon nanotubes in 1991 [1]. Following that the research on tungsten disulfide (WS2) [2], molybdenum disulfide (MoS2) [3], boron nitride (BN) [4], silicon oxide (SiO2) [5], and other nanotubes have also been reported. In the past two decades, researchers have conducted extensive experimental and theoretical researches on the materials with nanotube structures. This one-dimensional structure at the nanometer scale is favored by researchers because of its peculiar quantum effects [6,7], physical and chemical properties such as catalytic [[8], [9], [10]], luminescence [11,12], and transistors [[13], [14], [15], [16]]. MoS2 as a transition metal dichalcogenide (TMDs) has sandwich-type S–Mo–S hexagonal structure and exhibits excellent performance in various applications such as transistors [[17], [18], [19]], hydrogen evolution [[20], [21], [22]], and batteries [[23], [24], [25]]. Such excellent performance makes MoS2 expected to be the next generation of semiconductor materials after silicon and other potential applications.

The synthesis of MoS2 nanotubes by chemical transport reaction (CTR) method [26,27], chemical vapor deposition (CVD) method [[28], [29], [30]] and sulfurization of oxides [31] has been reported. Previously we have reported the use of FeO as a catalyst to synthesize MoS2 nanowires by CVD method [32]. We found that this nanowire is multi-walled tubular structure and exhibit a rectangular cross-section rather than a cylindrical shape [32]. This finding has initiated the computational study reported in this paper. The objective is two folds: One is to examine whether nanotubes with square cross section yields spontaneously. The other is to explore new properties of the square-nanotubes due to their unique shapes.

Section snippets

Computational details

This calculation based on DFT is done by using the Vienna ab-initio simulation package (VASP) [33]. The Projector Augmented Wave (PAW) potential was referred to the interaction between the atom core and valence electrons with the 500 eV as the plane-wave cutoff energy [34]. The band gap obtained by the GGA-PBE method for TMDs is in good agreement with the experimental values, since the energy gap can be accurately calculated [35]. In the unit cell, all the lattice parameters and atomic position

Geometric structure and energy

In the initial structure, square nanotube has n - 2 S atoms in the inner side and n + 2 S atoms in the outer side, which totals to 2n, while the number of Mo atoms is n. After full relaxation, the square nanotube has kept its square cross-section. When we set other number of S atoms in the outer and inner layers, such as both n for outer and inner layers, the structure relaxed to cylindrical one in some cases and tubular structure was destroyed in other cases.

The typical geometric structures of

Conclusion

In summary, we explored MoS2 nanotubes with square cross-section by using DFT calculation. The calculation result shows that this structure has lower strain energy in the low-diameter region (~30 Å) than the conventional cylindrical nanotubes. The surface energy per length of the square nanotubes remains high because of five coordinated Mo atoms at the corner. This makes the square nanotubes less stable than cylindrical nanotubes when the diameter is greater than 25 Å. Effect of some templates

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was partly supported by KAKENHI (17H03380) from MEXT, Japan. The computational work has been done using the facilities of the Supercomputer Center, the Institute for Solid State Physics, the University of Tokyo and Research Center for Computational Science, Okazaki, Japan.

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