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Séminaire QUEST

Jeudi 11 février à 9h30,
Salle Rémy Lemaire, K223

Orateur : Yannick Dappe (CEA-IRAMIS)
"Theoretical characterization of point defects on a MoS2 monolayer by Scanning Tunnelling Microscopy"

Abstract :

The possibility of isolating one monolayer of graphene in 2004 [1] has opened a wide field of research in the study of bidimensional (2D) materials. Among them, MoS2 is a very promising material whose electronic properties are strongly thickness-dependent. Indeed, the MoS2 bandgap evolves from indirect in its bulk phase to a bigger, direct bandgap as a monolayer [2]. This particular feature has raised a huge interest for the realisation of MoS2-based transistors, in particular in combination with graphene, the first one providing the required gap for an on/off electronics, the latter providing the fantastic electronic mobility. However, as it is commonly known for nanoelectronic devices, the performance of this material is strongly dependent on its crystal quality. As such, defects like vacancies or substitutional atoms can have a huge influence on the reactivity and transport properties of MoS2. Although the MoS2 monolayer has been intensively analysed with various experimental and theoretical techniques, so far only few recent works have presented a systematic Scanning Tunnelling Microscopy (STM) study of this system taking into account the potential point defects [3]. Here, we present an exhaustive characterisation of the Mo and S vacancies as well as a great variety of antisite cases, based on DFT calculations and STM image modelling, in order to explore the evolution of the MoS2 passivation and metallicity in the presence of point defects. Starting from the exact equation for the tunnelling current obtained within the Keldysh-Green’s functions formalism, our calculations confirm that the sulphur atoms dominate in the STM images for a wide range of voltages in the pristine monolayer. On the other hand, while the vacancies are imaged as depressions in the three cases here analysed (Mo, S and S di-vacancies), the substitutional atoms present different behaviours depending on the applied voltage. Our simulations thus provide a thorough theoretical characterisation of STM images of free-standing MoS2 monolayer in the presence of S and Mo vacancies and antisites, taking into account the full structural and electronic structure relaxation. Finally, we show how the inclusion of these point defects promotes the emergence of reactive dangling-bonds that can be efficient adsorption sites for external atoms or molecules [4].

References :
[1] K. S. Novoselov et al, Science 306, 666 (2004).
[2] K. F. Mak, C. Lee, J .Hone, J. Shan and T. F. Heinz, Phys. Rev. Lett. 105, 136805 (2010).
[3] K. C. Santosh, R. C. Longo, R. Addou, R. M. Wallace and K. Cho, Nanotechnology 25, 375703 (2014).
[4] C. González, B. Biel and Y. J. Dappe, accepted for publication in Nanotechnology (2016)

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