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On the origin of metallicity and stability of the metastable phase in chemically exfoliated MoS2

Amongst the non-graphene layered materials, transition metal dichalcogenides (TMDs) have been in the focus of the research community for quite a long time because of their fascinating properties and novel applications that emerge upon 2D confinement. Specifically, molybdenum disulphide (MoS2) in this series bears great promises for displaying wide range of electronic, mechanical, optical and chemical properties due to the presence of several polymorphs with distinctly different electronic structures including the possibility of exotic properties such as the quantum spin Hall effect. MoS2 structure is built from two close-packed planes of S2−ions, sandwiching the hexagonal arrangement of Mo4+ layer. Depending on the relative positioning of the two sulphur planes with respect to each other, two basic polymorphs of MoS2 are realised, with the H phase having the ABA arrangement and the T phase with the ABC arrangement of the S2−- Mo4+- S2− trilayers. While H is the thermodynamically ground state, octahedral T structure can undergo further distortion to minimize the energy, consistent with the simple molecular orbital picture. Such distortions lead to the formation of various superlattices (such as the so-called T’ and T” phases).
MoS2 can undergo phase transitions involving different routes, such as plasma hot electron transfer or chemical reactions. The chemical routes normally involve non-aqueous alkali metal intercalation followed by a water exfoliation step or sophisticated electrochemical control. Although theoretical calculations predict that different superlattices of the metastable state show different properties such as other bandgaps for ferroelectric insulator behaviour, it has been generally assumed that, all the above-mentioned processes lead to the formation of the metallic T phase. 
To investigate this system a team of researchers of the Indian Institute of Science performed spatially resolved XPS analysis by using the scanning photoelectron microscope hosted at the Escamicroscopy beamline at Elettra in collaboration with the local research team.
SPEM experiments have shown that Li+ions driven chemically exfoliated MoS2 has an extensive presence of a distorted metastable T’state that has a small bandgap compared to that of the stable H phase. Fig. 1-a) and b) show two photoemission intensity maps of Mo 3d region from MoS2 before (0W) and after (12W) water washing to remove the Liions; representative Mo 3d core levels of the two flakes are shown in panel c) compared with the typical spectrum of the H phase. Spectra near the Fermi edges for pristine H, 0W and 12W samples are plotted in panel d). The negligible intensity at the Fermi energy clearly suggests that none of these samples can be regarded as a metal. Raman spectra establish that this metastable state cannot be the undistorted T form, while the photoelectron spectroscopy directly shows the absence of any significant metallic density of states at the Fermi energy. Investigating samples with different extent of Liions (0W, 12W), it was shown that Li+ ions dope the small bandgap semiconducting metastable state to make these more conducting than in their pure state, explaining why such chemically exfoliated MoS2 has often been termed as metallic and this in turn has led to the erroneous conclusion of the undistorted T phase formation.
This work brings to conclusion the earlier investigation on this system at this beamline (see Pal et al., Phys. Rev. B 96, 195426 (2017).

Figure 1.    a) and b): photoemission intensity maps of Mo 3d region from MoS2 flakes with the full Li+ ions content (0W) and after water washing to remove the Li+ ions (12W); c) representative Mo 3d core levels of the two flakes compared with the typical spectrum of the H phase. d): Spectra near the Fermi edges for pristine H, 0W and 12W samples.
 

 

This research was conducted by the following research team:

Debasmita Pariari1, Rahul Mahavir Varma1, Maya N. Nair1, Patrick Zeller2, Matteo Amati2, Luca Gregoratti2, Karuna Kar Nanda3, D.D. Sarma1

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru, India
Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
Materials Research Centre, Indian Institute of Science, Bengaluru, India


Contact persons:

D.D: Sarma, email: sarma@iisc.ac.in
 

 

Reference

D. Pariari, R. M. Varma, M. N. Nair, P. Zeller, M. Amati, L. Gregoratti, K. K. Nanda, D.D. Sarma, “On the origin of metallicity and stability of the metastable phase in chemically exfoliated MoS2” Applied Materials Today 19, 100544 (2020), doi.org/10.1016/j.apmt.2019.100544



Last Updated on Thursday, 20 August 2020 18:22