Visic et al. Nanoscale Research Letters 2011, 6:593 http://www.nanoscalereslett.com/content/6/1/593 N A N O E X P R E S S Open Access Optical properties of exfoliated MoS coaxial 2 nanotubes - analogues of graphene Bojana Visic1*, Robert Dominko2†, Marta Klanjsek Gunde2†, Nina Hauptman2†, Sreco D Skapin1† and Maja Remskar1,3 Abstract We report on the first exfoliation of MoS2 coaxial nanotubes. The single-layer flakes, as the result of exfoliation, represent the transition metal dichalcogenides’ analogue of graphene. They show a very low degree of restacking in comparison with exfoliation of MoS2 plate-like crystals. MoS2 monolayers were investigated by means of electron and atomic force microscopies, showing their structure, and ultraviolet-visible spectrometry, revealing quantum confinement as the consequence of the nanoscale size in the z-direction. Keywords: molybdenum disulfide, nanotubes, exfoliation Background In recent years, significant progress has been made in exfoliating graphene directly from graphite, which is sup- posed to produce samples with fewer defects [1]. Exfolia- tion of the metallic-layered compounds TaS2 [2,3] and NbS2 [3] is known for more than 30 years. Also, prepara- tion of a single molecular layer of MoS2 out of the crystal- line 2H-MoS2 by intercalation of lithium has been reported in 1986 [4], which was the first exfoliation of a layered semiconductor, and it was followed by the exfolia- tion of WS2 [5]. Exfoliation via other solvents [6] and cleaving processes [7] has been reported recently. Until now, there have been no reports on the attempt to exfoli- ate transition-metal disulphide nanotubes. Bulk 2H-MoS2 is made of S-Mo-S sandwich layers, where every molybdenum sheet is between two sheets of sulfur. It was found that crystalline MoS2 has three poly- types: 1T, 2H, and 3R, where the integer indicates the number of layers per unit cell and T, H, and R indicate the trigonal, hexagonal, and rhombohedral primitive unit cells, respectively. Whereas the interactions within the sandwich correspond to the chemical bonds, the neighboring layers are weakly connected with Van der Waals bonds, and for- eign materials can be inserted into the Van der Waals gap, * Correspondence: bojana.visic@ijs.si † Contributed equally 1Jozef Stefan Institute, Jamova cesta 39, Ljubljana, 1000, Slovenia Full list of author information is available at the end of the article and under appropriate conditions, the layers can be further separated to form single molecular layers. Single molecular layers of MoS2 in a water suspension have been prepared by intercalation of lithium into crystal 2H-MoS2 followed by exfoliation in water. As this aqueous suspension is aging, restacked MoS2 with two monolayers of water is formed (the water-bilayer phase), with water monolayers between parallel, but rotationally disordered MoS2 layers. For this structure, a 2a0 × a0 pattern was confirmed [8], where a0 is the lattice constant of bulk 2H- MoS2. Single layer shows a change in lattice symmetry from 2H to 1T, and it is suggested that the change in coordination is electronically driven by Li electron dona- tion to the MoS2 host [9]; this configuration is preferred because the electrons donated to the valence band in 1T configuration occupy a much lower level than the elec- trons donated to the conduction band of the 2H structure. It was shown that this structural transition is followed by a change in the optical absorption spectrum, where two strong absorption peaks for 2H-MoS2 are absent [10]. The structural transformation is also present in the formation of single molecular layers of WS2. Lattice constants in the basal (001) plane were found: for 2H-MoS2 crystal with a trigonal prism configuration, it is 3.162 Å; for Li-MoS2 crystal with an octahedral configuration, 3.6 Å; and for MoS2 single layer with an octahedral configuration, 3.27 Å [11]. Exfoliation of MoS2 can lead to the synthesis of many new materials, obtained by restacking the single layers © 2011 Visic et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.