Unique Structural and Transport Properties of Molybdenum Chalcohalide Nanowires

We combine ab initio density functional and quantum transport calculations based on the nonequilibrium Green’s function formalism to compare structural, electronic, and transport properties of ${\mathrm{Mo}}_6{\mathrm{S}}_{6-x}{\mathrm{I}}_x$ nanowires with carbon nanotubes. We find systems with $x=2$ to be particularly stable and rigid, with their electronic structure and conductance close to that of metallic (13,13) single-wall carbon nanotubes. ${\mathrm{Mo}}_6{\mathrm{S}}_{6-x}{\mathrm{I}}_x$ nanowires are conductive irrespective of their structure, more easily separable than carbon nanotubes, and capable of forming ideal contacts to Au leads through thio groups.

Figure 1

(a) Schematic structure of a ${\mathrm{Mo}}_6{\mathrm{S}}_{6-x}{\mathrm{I}}_x$ nanowire in side and end-on view. (b) Energy gain $\Delta E(x)$ with respect to the average binding energy [23] and (c) energy $\Delta H(x)$ of the substitution reaction in Eq. (1) leading to the formation of a ${\mathrm{Mo}}_6{\mathrm{S}}_{6-x}{\mathrm{I}}_x$ nanowire as a function of composition. Among the data points for all structural isomers (○), the most stable structures are identified by the solid circles (•). (d) Deviation from the equilibrium binding energy $E_0$ as a function of the relative unit cell size $a/a_{\mathrm{eq}}$, for a segment of the magic ${\mathrm{Mo}}_6{\mathrm{S}}_4{\mathrm{I}}_2$ nanowire containing $N$ Mo atoms, and for an $N$-atom segment of the (13,13) carbon nanotube.

Figure 2

(a) Structure of ${\mathrm{Mo}}_6{\mathrm{S}}_4{\mathrm{I}}_2$ nanowires arranged on a simple hexagonal lattice in a plane normal to the wire axes. (b) Contour plot of the nanowire binding energy in this lattice as a function of the wire orientation $\phi$ and separation $d$. The energy is given in eV per formula unit.

Figure 3

Electronic properties of a ${\mathrm{Mo}}_6{\mathrm{S}}_{6-x}{\mathrm{I}}_x$ nanowire (a)–(f) in comparison to a (13,13) carbon nanotube (g)–(i). Displayed is the band structure $E(k)$ of the ${\mathrm{Mo}}_6{\mathrm{S}}_{6-x}{\mathrm{I}}_x$ nanowires in (a),(d), their DOS in (b),(e), and quantum conductance $G(E)$ in units of the conductance quantum $G_0$ (c),(f). The corresponding quantities for the (13,13) carbon nanotube are shown in (g)–(i). $E=0$, given by the dashed lines, corresponds to the Fermi level in (a),(b),(d),(e),(g),(h), and to zero source-drain voltage in (c),(f),(i). The dotted lines show the position of $E_F$ in externally gated or doped chalcohalide nanowires resembling carbon nanotubes.