Remskar et al. Nanoscale Res Lett 2011, 6:26 http://www.nanoscalereslett.com/content/6/1/26 Page 4 of 6 Figure 2 a X-ray powder diffraction pattern (A) The starting material composed of Mo6S4I6 nanowires with traces of MoS2 (asterisk) and Mo6S2I8. The two indexed peaks are used for approximation of the unit cell of Mo6S4I6; (B) The final material is composed of pure multiwall MoS2 nanotubes. b Raman spectra: (A) Mo6S2I8 nanowires, (B) Mo6S4I6 nanowires; arrows point traces of MoS2; square dot (see also the inset) shows a position of the characteristic peak at 117 cm-1, (C) MoS2 nanotubes gained by sulfurization of Mo6S4I6 nanowires. c Optical absorption spectra: (A) Mo6S4I6 precursor nanowires; (B) MoS2 nanotubes; (C) MoS2 polycrystalline material (powder). All materials were dispersed in ethanol. (Figure 2b-A). The only exception is the relative inten- sity of the peak at 117 cm-1, which is higher in the Mo6S4I6 compared to the Mo6S2I8. Some traces of MoS2 could also be observed in some spectra, marked by arrows. The spectroscopic similarity is a further support for the nearly identical crystal and local structure of Mo6S4I6 and Mo6S2I8. The Raman spectrum of the final product-MoS2 nanotubes (Figure 2b-C) contains the usual signature of MoS2, A1g mode at 409 cm-1 and E2g mode at 384 cm-1 [21]. No traces of precursor Mo6S4I6 nanowires are observed. A small peak at the 287 cm-1 is attributed to the MoS2, E1g mode that is forbidden in the backscattering experiments on the basal (001) plane. The reason for E1g occurrence is the cylindrical geome- try with a great part of the basal planes oriented in par- allel with the axis of illumination. The line widths of the MoS2 nanotubes are larger than that of MoS2 single crystals. The FWHM of the A1g line increases from 2.0 to 3.3 cm-1 and that of the E2g from 1.6 to 3.8 cm-1. The line broadening is attributed to a smaller crystallite size and to a larger amount of defects in multiwall MoS2 nanotubes compared to MoS2 single crystals [22,23]. The electronic band structure calculations show that the bulk MoS2 is an indirect gap semiconductor with two exciton absorption bands at the absorption edge [24]. The absorption associated with the direct band gap is located in the visible spectrum around 700 nm and results from a direct transition at the K point. Two peaks assigned as A1 (690 nm) and B1 (620 nm) are attributed to two excitons of the Rydberg series. The band at around 500 nm is associated with the direct transitions from the valence band to the conduction band [25]. UV-Vis absorption spectroscopy was performed at room temperature. The MoS2 powder, Mo6S4I6 nano- wires, and MoS2 nanotubes were ultra-sonicated in ethanol. MoS2 powder (Aldrich) was used as a refer- ence material. The UV-Vis absorption spectrum (Figure 2c) of the Mo6S4I6 nanowires (A) reveals two broad peaks at 748 nm (1.66 eV) and 487 nm (2.55 eV). The absorption spectrum of the MoS2 nanotubes (B) reveals the A1 peak occurring at 681 nm and B1 at 631 nm with energy separation of 0.14 eV. The third peak dominates the spectrum, and it is blue-shifted with respect to MoS2. It is composed of a peak at 472 nm (2.63 eV) and its shoulder at 416 nm (2.99 eV). Comparison with the spectrum obtained by dispersed MoS2 powder in the form of platelets (C) reveals two main differences in the MoS2 nanotube sample: (i) a decrease in energy separation between A1 and B1 associated with the spin-orbit splitting of the top of the valence gap at the K point [20] and (ii) a blue shift, relative intensity and the shape of the