Compositional ordering and quantum transport inMo6S9−xIxnanowires:Ab initiocalculations

Abstract

We use ab initio calculations to study the compositional ordering and quantum transport in Mo 6 S 9−x I x nanowires. The skeleton of these nanowires consists of Mo octahedra, which are functionalized by S and I adsorbates and connected by flexible S 3 bridges. The optimum geometries and relative stabilities at different compositions are determined by using density functional theory. We find nanowires with x = 3 to be particularly stable. Nanowires with other compositions are likely to phase separate into iodine-rich and iodine-depleted domains, some of which should have the Mo 6 S 6 I 3 stoichiometry. Our transport calculations, which are based on the nonequilibrium Green's function formalism, indicate that the nanowires are metallic independent of composition and exhibit a quantum conductance of G =3G 0 , with the three conductance channels involving the S 3 bridges.

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“…On the other hand, Mo 6 S 4.5 I 4.5 , with its relatively higher G, would be more interesting for composite fabrication, as a higher shear modulus would ensure better mechanical coupling between wires and the composite matrix. A surprising feature of the data is that the measured Young's modulus of the nanowire bundles is significantly larger than theoretically predicted [53,35]. Since agreement between theory and experiment is quite reasonable for other materials, the experimental data are considered reliable.…”

“…Vilfan [6] first calculated the stability of Mo 6 S 3 I 6 with the WIEN code starting with the structure reported by Meden et al [22] and with I atoms in the bridging positions and S and I on the outer ligand positions. Similar structural stability calculations were performed also by Tomanek's group using the SIESTA code [27], by Seifert et al [28,53], Ciraci et al [29], Murugan, et al [30] and by Sanvito [24].…”

“…We can therefore expect that after synthesis with a given stoichiometry the anion occupancies are to some extent random. Nevertheless, considering all possible configurations 0 6 x 6 6 in detail, Yang et al [28,53] find nanowires with x = 3 to be more stable than others. Since this does not correspond to the measured stoichiometries, the authors suggest that nanowires with compositions other than x = 3 may phase separate into iodine-rich and iodine-depleted domains, some of which should have the Mo 6 S 6 I 3 stoichiometry, such as are shown in Fig.…”

“…This suggests that the improved processing may lead to significant improvements in the intrinsic conductivity on the single wire level. In the absence of defects, DFT calculations of quantum conductance in the Landauer-Buttiker formalism [53] predict a metallic zero bias conduction of 1-3 G 0 per wire irrespective of S and I composition.…”

“…The second point of interest [35,27,53] is that the structure has a relatively easily deformable accordion-like link between adjacent clusters formed by the three S atoms, which leads to a very shal- low total energy potential as a function of the c lattice parameter (Fig. 16).…”