Effect of rapid thermal annealing in vacuum on the structural and optical properties of MoS 2 flakes in solution

Recently, top-down and bottom-up methods have been developed to fabricate single-layer (1L) MoS2 nanosheets. The top-down method focuses on the mechanical [1-2] and solution-based exfoliation [3-4] of bulk MoS2 crystals. Although the mechanical exfoliation of MoS2 can produce the pristine 1L MoS2 with high quality, its yield and reproducibility are low. The solution-based exfoliated MoS2 is often accompanied by residual chemicals from the solution used, which in turn affects the properties of MoS2 nanosheets [3]. In the previous studies, a thermal annealing method has been used in layer thinning and etching of mechanically exfoliated MoS2 for achieving single-layer MoS2 from multi-layer MoS2 [1]. In this work, MoS2 flakes of ~ 400 nm lateral size in ethanol solution were used to investigate the thermal annealing mechanism of MoS2 flakes. For single-layer MoS2 nanosheets, MoS2 flakes solution was dropped on the whole surface of 100 nm SiO2 substrate and subsequently, MoS2 flakes were heated by rapid thermal annealing at various temperatures from 100 to 500 o C under vacuum for 10 min. The annealed samples were characterized by optical microscopy, Raman spectroscopy, and atomic force microscopy (AFM). Fig. 1 shows the optical images of MoS2 nanosheets on 100 nm SiO2 /Si, in which different color contrast represents different layer thickness of MoS2. [5] Fig. 1 (a) shows the MoS2 flakes before thermal annealing, which consists mainly of 6L nanosheets. Subsequently, thermal annealing was performed at 200 °C for 10 min. The formation of 2L MoS2 nanosheet by thinning was observed after thermal annealing, as shown in Fig 1 (b). Raman spectra of the transferred layers exhibited two intense features, E 1 2g and A1g peaks at 384 and 403 cm -1 , respectively, uniquely characteristic of MoS2 film. The two Raman modes, E 1 2g and A1g, exhibited sensitive thickness dependences, with the frequency of the former decreasing and that of the latter increasing with thickness [6]. The annealing behaviors show several intriguing characteristics. Most strikingly, we find that the E 1 2g vibration softens (blue shifts), while the A1g vibration stiffens (red shifts) with increasing annealing temperature, as shown in Fig 2. This Raman analysis reveals optimum annealing temperature of 200 o C for the synthesis of smallest-layer-number and best-quality MoS2. The thickness of MoS2 estimated by AFM was consistent with the Raman results. As a result of thermal annealing, the MoS2 nanosheet is thinned, possibly due to its oxidation to form MoO3. Possible mechanisms are proposed to explain the formation processes of MoS2 nanosheets from MoS2 flakes during the annealing.