Visic et al. Nanoscale Research Letters 2011, 6:593 http://www.nanoscalereslett.com/content/6/1/593 with, for example, organic molecules [12-14]. It was dis- covered that MoS2 photoluminescence increases with decreasing layer thickness, the strongest is on single layer [15], which holds promise for new nanophotonic applications, and it was also realized as a field-effect transistor [16], which can be applied in new areas of optoelectronics. MoS2 is also known as a solid lubricant which has been used in the industry for the last 60 years. At low-humidity conditions, it is possible to obtain a low friction coefficient of 0.05 [17]. Ultra-low friction of 0.003 was reported between MoS2 flakes and MoS2 surfaces [18]. The pro- blem of edge oxidation and preservation of the flakes in parallel orientation with the surface with a low degree of restacking can be minimized with the reduction of thick- ness. It is desired to obtain the thinnest flakes possible, which can be achieved by exfoliating MoS2 coaxial nanotubes. Methods The MoS2 coaxial nanotubes (Nanotul Ltd., Ljubljana, Slovenia) are synthesized by sulfurization transformation of M6S2I8 nanowires under gas flow of H2/H2S mixture in an argon atmosphere [19]. They were dried in a dry box (glove box, < 1 ppm of H2O, M.Braun Garching, Germany) for at least 6 h in vacuum at 120°C, then sus- pended in a solution of 2.5 M butyllithium in hexanes (0.693 g/mL, Sigma-Aldrich, St. Louis, MO, USA), where it was left for 3 days. Exfoliation occurs by immersing the lithium-intercalated compounds in water after taking them out of the dry box, which provides a water-bilayer phase of MoS2. To obtain single layers of MoS2 in water suspension, the material was washed repeatedly with dis- tilled water and centrifuged. The reaction that occurs between the water and intercalated lithium results in hydrogen gas release and lithium hydroxide formation. The washing process reduces lithium concentration (from a pH of 12 to 7). Consequently, the water-bilayer phase, which is stable in a higher pH [5], splits into single MoS2 layers. The exfoliated material was characterized by scanning electron microscopy [SEM], transmission electron micro- scopy [TEM], atomic probe techniques (atomic force microscopy [AFM] and STM), and X-ray diffraction [XRD]. The XRD spectra were recorded with an AXS D4 Endeavor diffractometer (Bruker Corporation, Karlsruhe, Germany), with Cu Ka1 radiation and a SOL-X energy- dispersive detector with the angular range of 2θ from 5° to 75° with a step size of 0.04° and a collection time of 3 to 4 s. The process of exfoliation was elucidated by ultraviolet- visible [UV-Vis] spectroscopy. The spectra were recorded in a 10-mm-path length quartz cell on an Agilent 8453 UV-Vis Spectrophotometer (Agilent Technologies, Inc., Page 2 of 6 Santa Clara, CA, USA) at 23°C ± 1°C in a wavelength range of 180 to 1,000 nm, with a 1-nm resolution. For comparison, the PerkinElmer Lambda 950 photospect- rometer (Waltham, MA, USA) was used under the same conditions. All UV-Vis measurements were performed on the material in a water solution. Results and discussion The main reason why the nanotubes (Figure 1a) are used for exfoliation is that they already have gaps between their coaxial cylinders, as shown in Figure 1b. The nanotubes keep an outside shape of the Mo6S2I8 nanowire precursor, but the difference in mass density between the wires and MoS2 compounds leads to a creation of an empty space inside the MoS2 nanotubes that separates the adjacent cylinders and creates gaps between them [19], so it is easier to intercalate them; and the nanotube walls have a curvature, which prevents restacking. The TEM micrographs of exfoliated MoS2 nanotubes are shown in Figures 1c, d, while the AFM image and the corresponding profile are shown in Figures 1e, f, respec- tively. The final product of the exfoliation process contains mainly of single-layer MoS2 flakes. The UV-Vis absorption spectra were measured on exfo- liated MoS2 nanotubes, and the comparison was made with the MoS2 coaxial nanotubes (used for exfoliation) and 2H-MoS2 plate-like powder (< 2 nm, 99% purity, Sigma-Aldrich, St. Louis, MO, USA). The samples were prepared in a form of dispersion in water, with concentra- tions showing comparable intensities of optical absorption. Both powder and nanotube spectra (Figure 2) show the features that can be assigned to the A and B excitons, characteristic for the 2H-polytype and correspond to the smallest direct transition at the K point of the Brillouin zone (K4 ® K5 and K1 ® K5 transitions, respectively) [20]. The existence of the two excitons is due to the interlayer interaction and spin-orbit splitting, with the splitting value of approximately 60 nm (0.17 eV). For the MoS2 coaxial nanotubes, peak positions for A and B excitons are at 702 nm (1.77 eV) and 644 nm (1.92 eV), respectively, with the red shift of the absorption peaks compared to the powder, where their values are 692 nm (1.79 eV) and 634 nm (1.96 eV). The red shift is due to the quantum confinement, as explained by Frey et al. [21]. The excitons are separated by 60 nm for both materials, which is in good agreement with the literature [21]. Another broader peak, observed at 540 nm (2.30 eV), can be assigned to a direct transition between the states deep in the valence band to the conduction band at the M point of the Brillouin zone [20]. The strong peak at 210 nm (5.9 eV), being at the edge of the spectrometer’s range, is usually disregarded from the analysis, for the wave- lengths were so small, we get increased scattering, and one of the consequences is a false peak. Since the energy