The intrinsic defect structure of exfoliated MoS2 single layers revealed by Scanning Tunneling Microscopy.

Péter Vancsó,Ji-Young Noh,Chanyong Hwang,László P Biró,Levente Tapasztó,Yong-Sung Kim,Gábor Zsolt Magda,János Pető

doi:10.1038/srep29726

Abstract

MoS2 single layers have recently emerged as strong competitors of graphene in electronic and optoelectronic device applications due to their intrinsic direct bandgap. However, transport measurements reveal the crucial role of defect-induced electronic states, pointing out the fundamental importance of characterizing their intrinsic defect structure. Transmission Electron Microscopy (TEM) is able to image atomic scale defects in MoS2 single layers, but the imaged defect structure is far from the one probed in the electronic devices, as the defect density and distribution are substantially altered during the TEM imaging. Here, we report that under special imaging conditions, STM measurements can fully resolve the native atomic scale defect structure of MoS2 single layers. Our STM investigations clearly resolve a high intrinsic concentration of individual sulfur atom vacancies, and experimentally identify the nature of the defect induced electronic mid-gap states, by combining topographic STM images with ab intio calculations. Experimental data on the intrinsic defect structure and the associated defect-bound electronic states that can be directly used for the interpretation of transport measurements are essential to fully understand the operation, reliability and performance limitations of realistic electronic devices based on MoS2 single layers.

Highlights

Beyond graphene, alternative two-dimensional materials, such as transition-metal dichalcogenides (TMDCs), are in the focus of scientific attention due to their intrinsic direct bandgap and various intriguing properties[1,2]
The critical role of defects in the electronic behavior of MoS2 single layer devices is clearly demonstrated by Qiu et al.[10] who reported a novel defect-mediated transport mechanism where the transport is dominated by hopping via defect induced localized states
The fact that in our STM measurements the position of V2S defect has not changed during repeated scanning, namely no rotation has been observed, further strengthens our previous observation that the native point defects of MoS2 single layers are immobile at room temperature and STM does not perturb the intrinsic defect structure of MoS2 single layers

Summary

Results and Discussion

In the experimental STM images (Fig. 3a), besides the individual S vacancies, pairs of triangular shape defects (marked by white circles) have been observed, which form disulfur vacancies (V2S’s) To confirm this observation we have calculated the STM image of a sulfur divacancy using DFT calculations where two neighboring S atoms from the top S layer are removed. The good agreement of the simulated STM image (Fig. 3b) of the (a1 −a1)* state with the experimental observations (Fig. 3a) confirms this expectation and the presence of native disulfur vacancies (V2S) in our single-layer MoS2 sample Their concentration was experimentally found to be much lower as compared to single S atom vacancies. The fact that in our STM measurements the position of V2S defect has not changed during repeated scanning, namely no rotation has been observed, further strengthens our previous observation that the native point defects of MoS2 single layers are immobile at room temperature and STM does not perturb the intrinsic defect structure of MoS2 single layers

Conclusions

Author Contributions

Additional Information