Optical properties of ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ nanowires

Optical reflectivity and absorbance measurements of oriented ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ nanowire thin films and dispersions in different solvents are presented extending from the far infrared to the ultraviolet. In spite of the highly one-dimensional character of the nanowire material and narrow electronic valence and conduction subbands, as predicted by the density-functional theory calculations, sharp Van Hove features in the optical absorption spectra are not observed, partly because of the large density of interpenetrating electron subbands and partly due to damping and disorder. The optically measured electrical conductivity extrapolated to zero frequency ${\sigma }_1(\omega \to 0)$ and the calculated conductivity are significantly higher than the typical dc value from resistance measurements, indicating that disorder limits electron transport, a feature characteristic of strongly one-dimensional systems.

Figure 1

(Color online) The atomic structure of ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ nanowires. (a) Side view of an individual ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ molecular chain with S atoms in the bridging positions and (b) projection along the crystalline $c$ axis. Early x-ray diffraction experiments indicated chain ordering according to the hexagonal $P{6}_3$ space group (Ref. 12) (the unit cell shown with solid lines), whereas the most recent electron microscopy experiments suggest the $P\overline{1}$ space group (Ref. 13) (dashed lines). The essential difference between the two space groups is the stacking, i.e., mutual displacements of individual molecular chains along the $c$ axis.

Figure 2

(Color online) (a) A SEM image of the oriented ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ film on quartz and (b) an AFM image of the oriented ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ film on quartz. The image size is $10\times 10\mu {\mathrm{m}}^2$. The surface roughness is approximately $8\mathrm{nm}$, and the average film thickness is approximately $50\mathrm{nm}$.

Figure 3

(Color online) (Top) The reflectivities $R\left(\omega \right)$ of annealed (sample 1) and unannealed (sample 2) samples and the calculated $R\left(\omega \right)$ for two isomers (3S and 3I) and random polarizations. (Bottom) The optical conductivity ${\sigma }_1\left(\omega \right)$ of the two samples, derived from $R\left(\omega \right)$, and the calculated ${\sigma }_1\left(\omega \right)$, multiplied by 0.1. In this and the following figures, the isomer 3S has S atoms in the bridging positions, whereas in the case labeled 3I, the bridging atoms are I, and the S atoms occupy the dressing positions. In the simulations, the intraband transitions (Drude peak) are damped with $\Gamma =0.5\mathrm{eV}$ and the interband transitions with $\Gamma =0.1\mathrm{eV}$.

Figure 4

(Color online) The calculated complex refractive index $(n,k)$ and conductivity ${\sigma }_1$ for the 3S structure and for random polarizations. The solid circle is an ellipsometrically measured value of the refractive index $n$ at $1.53\mathrm{eV}$ (Ref. 31).

Figure 5

(Color online) Optical absorbance in oriented thin films. Top panel refers to the polarization parallel to the long molecular axis and bottom panel to the perpendicular polarization. Solid lines are from the experiment (arbitrary scale), and the other lines are calculated in the RPA.

Figure 6

(Color online) The absorption spectra of ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ nanowires in different solvents. ${\mathrm{Mo}}_6{\mathrm{S}}_{4.5}{\mathrm{I}}_{4.5}$ in IPA is shown for comparison from Ref. 34.

Figure 7

The absorption spectra in the near infrared region of different ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ sediments redispersed in IPA (solid lines). Pure IPA is shown for comparison (dotted line).

Figure 8

(Color online) The absorption spectra of ${\mathrm{Mo}}_6{\mathrm{S}}_3{\mathrm{I}}_6$ nanowires in ethanol and measured on a thin film.