Visic et al. Nanoscale Research Letters 2011, 6:593 http://www.nanoscalereslett.com/content/6/1/593 Page 4 of 6 Figure 2 UV-Vis spectra of the powder and nanotubes. UV-Vis absorption spectra of the (a) MoS2 powder and (b) MoS2 nanotubes. associated to this peak is too large to be indubitably assigned to a particular electronic transition, the nature of the peak is still inconclusive. The change from Li-MoS2 to exfoliated MoS2 was recorded by UV-Vis absorption during a centrifugation- washing process (Figure 3). At the beginning, every vial consists of 0.5 mL of Li-MoS2, diluted with 1.5 mL of dis- tilled water and sonicated for 5 min. The UV-Vis spec- trum of the initial solution is shown in Figure 3a. Each vial was centrifuged for 20 min at 10,000 rpm, and the liquid part was replaced with distilled water in order to remove the excess lithium. The process has to be repeated at least five times, during which the pH is lowered from 12 to 7, and thus, the single layers are obtained from the removed liquid. The corresponding spectrum is shown in Figure 3b. Both the Li-MoS2 and the exfoliated MoS2 have peaks around 200 nm as well as two peaks around 300 and 400 nm (4.11 and 3.11 eV, respectively), as seen in Figure 3. There is no evidence of excitons at approxi- mately 700 nm. We suggest that the two peaks asso- ciated with the exfoliated material are A and B excitons that exhibit a large blueshift (2.23 and 1.05 eV, respec- tively) due to the quantum-size confinement. To assert this claim, the effective mass treatment was applied [22]. This model is used to describe size quantization of the carriers’ energy spectrum in semiconductors. In terms of the model, and in the size regime where quantum con- finement effects are prominent, the shift of the absorp- tion edge or bandgap is inversely proportional to the effective mass of the excitons: Eg ≈ (πμLh¯z2)2 , where ΔEg is the energy shift, μ is the excitons’ effec- tive mass in the direction parallel to the z-axis, and Lz is the thickness of the nanoparticles in the z-direction. For the given exciton masses, μA = 1.28 me and μB = 4.10 me [23], and energy shift obtained in our experi- ment, we can estimate the thickness of the sample: LzA ≈ 5 A˚ and LzB ≈ 4 A˚ . Both values are in the frame of accuracy for the MoS2 monolayer thickness [11]. 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 Figure 3 Aging. Absorption spectra of MoS2 during the exfoliation process: (a) the initial sample containing Li-MoS2; (b) the completely exfoliated MoS2 nanotubes; (c) the completely exfoliated MoS2 nanotubes after 29 days. Figure 4 XRD of the nanotubes. X-ray spectrum of the MoS2 coaxial nanotubes.