Remskar et al. Nanoscale Res Lett 2011, 6:26 http://www.nanoscalereslett.com/content/6/1/26 template method [15], hydrothermal synthesis [16] and chemical transport reaction using iodine as a transport agent [17]. Currently, most of the described techniques are not suitable for large-scale production of pure multi- wall MoS2 nanotubes, which would possess a relatively uniform size. In the present paper, we report on a synthesis that can be scaled up for bulk production of pure multiwall MoS2 nanotubes of lengths up to several tens micro- meters and diameters up to 100 nm using groups of Mo6S4I6 nanowires as precursor crystals. The structural data are combined with optical absorbance and Raman scattering. In addition, two-probe current–voltage mea- surements were performed on a single nanotube. Experimental Sample Preparation The Mo6S4I6 nanowires were fabricated in evacuated (10-4 Pa) quartz ampoules directly from molybdenum and sulfur powder (Aldrich, 99.98 %), and iodine flakes (99.999%, Aldrich) in a molar ratio of 6: 3: 9. The iodine was used as the transport agent in the chemical trans- port reaction, which took place in a two-zone horizontal furnace for 48 h under a temperature gradient of 5.5 K/cm. A fraction (5–10 wt.%) of the total synthesized material was transported to the low-temperature zone (1010 K) of the ampoule and grew in the form of a hairy foil composed of Mo6SxIy nanowires with some traces of MoI2 at the interface with the quartz, while the material remaining at the hot zone (1123 K) appeared as a dark-brown powder. The stoichiometry of this remaining powder in the form of Mo6S4I6 nanowires was determined by wave dispersive analysis using a scanning electron microscope, Jeol JSM 6500F. These nanowires were used as the precursor mate- rial for transformation into MoS2 nanotubes by annealing at 1073 K in the reactive gas composed of 98 vol% of Ar, 1 vol% of H2S, and 1 vol% of H2 for 1 h. In a typical experi- ment, around 600 mg of the starting material was sulfur- ized and transformed into MoS2 nanotubes. The total mass of the starting material during the transformation was decreased for 40% due to the complete removal of iodine and its substitution by the lighter sulfur. X-ray powder dif- fraction and X-ray energy dispersive analysis of the end product reveal the iodine-free MoS2 compound. Characterization The Mo6S4I6 precursor crystals and MoS2 nanotubes have been studied by high-resolution 200 keV Jeol 2010 F field-emission transmission electron microscopes (HRTEM) and scanning electron microscope FE-SEM, Supra 35 VP, Carl Zeiss. X-ray diffraction (XRD), optical absorption, Raman spectroscopy, and wave dispersive analysis (WDS) were used to characterize the obtained materials. X-ray diffraction (XRD) was performed at Page 2 of 6 room temperature with a D4 Endeavor diffractometer (Bruker AXS) using quartz monochromator Cu Ka1 radiation source (l = 0.1541 nm) and Sol-X energy dis- persive detector. Angular range 2 θ was chosen from 6° to 73° with a step size of 0.04° and a collection time of 4 s. The samples were rotated during measurements at 6 rpm. Raman spectra were recorded in a micro-Raman 180° backscattering configuration on a Labram HR spec- trometer with a spectral resolution of 1.5 cm-1 deter- mined by the width of 3 CCD-pixels. For excitation, a frequency-doubled Nd:YAG 532 nm laser operated with 100 μW power on the sample was used. Under these conditions, heating or degradation effects were excluded. Transport properties were measured using an Agilent 4155 semiconductor parameter analyzer using on-wafer probing of two-terminal test structures. Results and Discussion The Mo6S4I6 Nanowires Mo6S4I6 nanowires grew as hedgehog-like self- assemblies (Figure 1a) composed of nanowires of very homogeneous size, up to 100 nm in diameter and up to 20 μm in length. Considering that little information is available about this phase with no unit cell determined [18], we describe the direction of growth and assign- ment of the diffraction pattern in accordance with the similar Mo6S2I8 phase [19]. We find close similarities of electron and X-ray diffraction patterns of both phases, which generalize the report [20] on the stability of the Mo6S9-xIx nanowires (4.5 < x < 6) with different S and I stoichiometries, to the Mo6S4I6 phase. Nanowires of dif- ferent stoichiometries grow in skeletal structures com- posed of one-dimensional polymer chains of Mo6– chalcogen–halogen clusters, which differ only in the site occupation by sulfur and iodine. This makes difficulties in the determination of a particular phase, especially based on X-ray results. In our studies, we used electron diffraction obtained on a single nanowire for the eluci- dation of the symmetry rules, X-ray diffraction for the determination of interlayer distance with sufficient accu- racy, and wave dispersive analysis for the determination of the stoichiometry on a single nanowire. Due to a mixed range of selective area diffraction, one cannot exclude the presence of other Mo6S9-xIx and Mo6S10-xIx nanowires in the starting materials, like Mo6S3I6 or Mo6S2I8 [20]. Nevertheless, most of the starting materi- als can be attributed to one phase, i.e. Mo6S4I6, while the others incorporate impurities that cause broadening of the X-ray peaks. The Mo6S4I6 nanowires possess a high aspect ratio and grow in a longitudinal direction along the [001]. The needles are rigid and well crystallized (Figure 1b). One-dimensional chains are mutually ordered and in contrast with reported Mo6S3I6 nanowires do not