Stability and electronic properties of α/β-Mo6S6 nanowires encapsulated inside carbon nanotubes
Introduction
One-dimensional (1D) material, which could be obtained by etching or crimping 2D materials, is one of the hot spots in present nanoscience and nanotechnology. Similar to 2D materials, one-dimensional materials host excellent electronic, optical, mechanical, and magnetic properties, which foreshows the bright application prospects in the fields of electronic devices [[1], [2], [3], [4]] [[1], [2], [3], [4]] [[1], [2], [3], [4]]. Meanwhile, owing to the further reduction of dimensions, one-dimensional materials possess larger specific surface area and more obvious quantum confinement effect, which endows them with potential applications in the field of catalysis. Taking carbon nanotube (CNT) as an example, the rehybridization between in-plane σ-bonding and out-of-plane π-bonding makes this representative 1D material electrically and thermally more conductive, mechanically stronger, and chemically and biologically more active [5,6].
Analogous to CNTs, transition metal chalcogenide (TMC) with stoichiometric formula M6X6 (M = Mo or W; X = S, Se or Te) is also a crucial one among the numerous 1D materials [[7], [8], [9]] [[7], [8], [9]] [[7], [8], [9]]. Generally, the M6X6 (M = Mo or W; X = S or Se) nanowires without Te element are metallic [10,11], which endues them with a natural advantage as a conducting wire. The TMC nanowires with Te element, however, are predicted to possess intrinsic semiconducting property [12]. Based on these features, one might construct a whole TMC nanowires electronic circuit without any other doping elements, and thus leading to a widespread attention on such distinctive 1D material. Meanwhile, the experimental preparation of TMC nanowires has also made progress recently. For instance, by using physical and chemical vapor deposition method, J. Kibsgaard et al. and L. Venkataraman et al. successfully synthesized Mo6S6 and Mo6Se6 nanowires [13,14]. Epitaxial growth is also an effective method to obtain TMC nanowires. Through using this method D. Le et al. fabricated Mo6S6 nanowires on Cu (1 1 1) substrate [15]. Moreover, numerous TMC nanowires have also been prepared by dissolving, vacuum annealing, and electron beam etching of corresponding 2D TMC [10,12,16,17]. Nevertheless, it is regrettable that the TMC nanowires obtained from the previous treatments are usually in bundles or mixed with bulk TMCs, which greatly limits the fascinating physical properties held by isolated TMC nanowires.
In order to break such drawback, great efforts are made to realize high-quality, large-scale isolated TMC nanowires. Through doping iodine atoms, isolated Mo6S6-type nanowires with the chemical formula Mo6SxIy could be prepared successfully [[18], [19], [20], [21]] [[18], [19], [20], [21]] [[18], [19], [20], [21]]. Nevertheless, the electronic performance of pure Mo6S6 nanowires is greatly changed due to the introduction of iodine atoms. That is to say, the introduction of iodine might be not a suitable approach for isolating Mo6S6 nanowires. Recently, utilizing carbon nanotubes as shielding, M. Nagata et al. developed a simple bottom-up synthesis method to isolate Mo6Te6 nanowires with definite atomic structure [22]. Compared with the non-encapsulated nanowire structure, such composite structure facilitates producing a large number of isolated nanowires under the protection of CNTs, which provides a reliable material platform for the practical applications of TMC nanowires. Unlike the breakthrough in the experiment, there still exists some unexplored and urgent theoretical issues about this composite structure, e.g., what diameter of CNT is the most suitable for TMC nanowires? Whether the original electronic properties of TMC nanowires could be kept well even with the interaction of the CNTs?
Motivated from the above analysis, in this paper, we systematically calculated the geometric structure and electronic band structure of two different phases of Mo6S6 nanowires encapsulated inside different CNTs. Our calculations show that due to the protection of carbon nanotubes, two phases of Mo6S6 are stable in the CNT, similar to the observation in experimental [22]. Meanwhile, the optimal chirality indices of CNT for α-Mo6S6 and β-Mo6S6 are (16,0) and (9,9) respectively. It is also found that despite the introduction of CNTs, the intrinsic electronic properties of the inner Mo6S6 are still preserved. Additionally, there is weak charge transfer from CNT to Mo6S6. The findings presented in this paper shared light on the structure and electronic properties of Mo6S6 nanowires in CNT, which could provide some guidelines for further step of TMC nanowires on the road of practical applications.
Section snippets
Methods
The first-principles calculations were performed with Vienna ab initio Simulation Package (VASP) [23,24] basing on density functional theory and using the Projector augmented-wave (PAW) pseudo-potentials to account for electron-ion interactions [25]. All structures in our consideration were optimized until all the Hellman-Feynman forces on each ion fell below the threshold value of 0.01 eV/Å by using the generalized gradient approximation (GGA) [26] with the Perdew-Burke- Ernzerhof (PBE) [27,28
Results and discussion
In our article, we consider two phases of Mo6S6 nanowire. As shown in Fig. 1(a) and (b), no matter what kind of phase of Mo6S6 nanowire, the molybdenum atoms from octahedral units in the chain are acted as backbone of nanowire and capped by sulfur atoms at the boundary. The main geometric difference between these two phases comes from the S and Mo atomic layers. For the α-Mo6S6 phase, the S and Mo atomic layers are both constrained within single layers, whereas for the β-Mo6S6 phase, the S and
Conclusion
In summary, we systematically investigate the geometries and electronic properties of two phases of Mo6S6 encapsulated inside CNTs in terms of the first principles calculations. Our results indicate that the α-Mo6S6 in different CNTs can always exist stably because of the protection of carbon nanotubes, and the optimal chirality index of CNT is (16,0). As for the β phase of Mo6S6 nanowires, only CNT (9,9) could ensure its structural stability. Moreover, the encapsulation of CNT plays a weak
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.
Acknowledgement
This work is supported by the National Natural Science Foundation of China (No. 11774298, 52073243).
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