Researchers Investigated Properties of MoS2/MoSi2N4 Heterostructure (Material Science)

Recently, the discovery of graphene by Andre Geim, who has received the 2010 Nobel prize in physics, a new era has been opened in the electronics and optoelectronics fields. Graphene is a 2D nanostructured material with outstanding electronic and mechanical properties. After that, scientists started to follow 2D graphene and discovery of other 2D nanosheets using different elements, for example, Mxenes, perovskites, transition metals dichalcogenides, and so on with excellent electronic, optical, mechanical, and thermoelectric properties both experimentally and theoretically. 

“Our MoS2/MoSi2N4 work was inspired by the paper, which has been recently published in the journal of science with the title of “Chemical vapor deposition of layered two-dimensional MoSi2N4 materials”. In this paper, we made a heterostructure using MoS2 and MoSi2N4 structures to improve electronic and optical properties, with minimum lattice mismatch between them, by the means of the first-principle calculations.”

— told M. Faraji, lead author of the study.

Now, Faraji and colleagues, in their recent study, introduce a novel van der Waals heterostructure of MoS2/MoSi2N4. They investigated the structural, electronic and optical properties of MoS2/MoSi2N4 heterostructure (HTS) by using first–principle calculations and verified its stability by calculating phonon dispersion curves and thermodynamic binding energy calculations. Their study recently appeared in New Journal of Chemistry.

Fig 1: Potential average per distance © Faraji et al.

They found that heterostructure have asymmetric electrostatic potential. While, the calculated work function for the HTS is 5.04 eV, which is smaller than the corresponding value of MoSi2N4 and MoS2 monolayers. This means that the electrons in the heterostructure have the ability to escape from the surface more than its individual monolayer surfaces under applied field.

Fig 2. Phonon band dispersion for MoS2/MoSi2N4 heterostructure. © Faraji et al.

To investigate the dynamical stability of heterostructure, they calculated phonon dispersion curves (as shown in fig 2 above) and found that phonon branches are free from any imaginary frequencies indicating the dynamical stability of the heterostructure.

Fig 3. Electronic band structure of MS/MSiN © Faraji et al.

They also investigated electronic properties of the heterostructure and found that due to the interaction between the two monolayers, the valance band maximum (VBM) of HTS is very similar to the corresponding value of MoS2 monolayer. On the other hand, the conduction band maximum (CBM) of HTS is lower than the corresponding value of MoS2 and MoSi2N4 monolayers. Therefore the bandgap of HTS (i.e. 1.26 eV) is smaller than the bandgap of its individual monolayers.

Furthermore, while investigating the optical properties, they found that the absorption values of the HTS are higher than the corresponding values of MoS2 and MoSi2N4 monolayer in the range of 0-300 nm for x and y components and in the whole range in the z component (refer fig 4 (c)). Therefore the HTS can enhance the absorption of light not only in the ultraviolet region but also in the visible region.

Fig. 4 (a) Real part of dielectric constant, (b) imaginary part of dielectric constant, (c) absorption and (d) refractive index of the MoS2, MoSi2N4 and MoS2/MoSi2N4 HTS at different applied electric field. © Faraji et al.

Finally, they manifested that the refractive index of heterostructure can be understood in terms of a combination of the refractive indexes of its monolayers. They showed that the combination between the refractive indices of MoS2 and MoSi2N4 monolayers gives the refractive index of the HTS in all directions. Therefore the refractive index of the HTS is larger than the refractive index of its monolayers (MLs) in the shown wavelength range.

Reference: Asadollah Bafekry, Mehrdad Faraji, Ali Abdollahzadeh, Mohamed Fadlallah, Chuong Nguyen, Mitra Ghergherehchi and S.A.H Feghhi, “Van der Waals heterostructure of MoS2/MoSi2N4: A First-principle study”, New Journal of Chemistry, 2021. Link to the paper

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