Optical properties of exfoliated MoS2 coaxial nanotubes - analogues of graphene
- Bojana Visic1Email author,
- Robert Dominko†2,
- Marta Klanjsek Gunde†2,
- Nina Hauptman†2,
- Sreco D Skapin†1 and
- Maja Remskar1, 3
© Visic et al; licensee Springer. 2011
Received: 31 August 2011
Accepted: 15 November 2011
Published: 15 November 2011
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.
In recent years, significant progress has been made in exfoliating graphene directly from graphite, which is supposed to produce samples with fewer defects . Exfoliation of the metallic-layered compounds TaS2 [2, 3] and NbS2  is known for more than 30 years. Also, preparation of a single molecular layer of MoS2 out of the crystalline 2H-MoS2 by intercalation of lithium has been reported in 1986 , which was the first exfoliation of a layered semiconductor, and it was followed by the exfoliation of WS2 . Exfoliation via other solvents  and cleaving processes  has been reported recently. Until now, there have been no reports on the attempt to exfoliate 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 polytypes: 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 foreign materials can be inserted into the Van der Waals gap, 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 2a 0 × a 0 pattern was confirmed , where a 0 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 donation to the MoS2 host ; this configuration is preferred because the electrons donated to the valence band in 1T configuration occupy a much lower level than the electrons 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 . 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 .
Exfoliation of MoS2 can lead to the synthesis of many new materials, obtained by restacking the single layers with, for example, organic molecules [12–14]. It was discovered that MoS2 photoluminescence increases with decreasing layer thickness, the strongest is on single layer , which holds promise for new nanophotonic applications, and it was also realized as a field-effect transistor , 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 . Ultra-low friction of 0.003 was reported between MoS2 flakes and MoS2 surfaces . The problem 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 thickness. It is desired to obtain the thinnest flakes possible, which can be achieved by exfoliating MoS2 coaxial nanotubes.
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 . 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 suspended 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 distilled 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 , splits into single MoS2 layers.
The exfoliated material was characterized by scanning electron microscopy [SEM], transmission electron microscopy [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 Kα1 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., 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 photospectrometer (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 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, respectively. The final product of the exfoliation process contains mainly of single-layer MoS2 flakes.
The UV-Vis absorption spectra were measured on exfoliated 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 concentrations showing comparable intensities of optical absorption.
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. . The excitons are separated by 60 nm for both materials, which is in good agreement with the literature . 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 . 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 wavelengths were so small, we get increased scattering, and one of the consequences is a false peak. Since the energy associated to this peak is too large to be indubitably assigned to a particular electronic transition, the nature of the peak is still inconclusive.
where ΔE g is the energy shift, μ is the excitons' effective mass in the direction parallel to the z-axis, and L z is the thickness of the nanoparticles in the z-direction.
For the given exciton masses, μ A = 1.28 m e and μ B = 4.10 m e , and energy shift obtained in our experiment, we can estimate the thickness of the sample: and . Both values are in the frame of accuracy for the MoS2 monolayer thickness .
The aging process of the exfoliated material was observed, as shown in Figure 3c. The main feature is that the peak at 200 nm becomes more prominent in time, but the shoulders at 300 and 400 nm remain unaltered. For the exfoliated bulk material, the evidence of restacking starts to occur in a few days with the reappearing of A and B excitons at 700 nm . On the contrary, for the exfoliated nanotubes, this effect was not observed even after 3 months.
The calculated effective particle size of the given reflections
Effective particle size
MoS 2 nanotubes
Exfoliated MoS 2
MoS 2 nanotubes
Exfoliated MoS 2
15 ± 1
2 ± 1
7 ± 1
5 ± 1
3 ± 1
5 ± 1
3 ± 1
30 ± 1
15 ± 1
Exfoliated MoS2 coaxial nanotubes are produced via chemical exfoliation, resulting in single-layer flakes that are stable for months, with a low degree of restacking. Both X-ray spectra and TEM images confirm that the material is indeed composed of MoS2 monolayers. In addition, UV-Vis spectra show a strong quantum confinement effects. The relatively simple process of getting one-layer-thick MoS2 can be used to provide new types of materials with possible applications in polymer composites, photovoltaics, and nanoelectronics.
The authors thank Janez Jelenc for the AFM images and Janez Kovac for the useful discussions.
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