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MoS2 is a compound in transition metal dichalcogenide (TMDC) family that is semiconductor with layered honeycomb structure (2H) having strong in-plane bonding and weak out-of-plane van der Waals (VDW) interactions. In the bulk form, MoS2 has an indirect band gap where valence band maximum and conduction band minimum are located at the Γ point and the middle point between Γ-K point, respectively. Alternatively, the monolayer form has direct band gap (at the K point) which is more suitable for device applications. However, exfoliation of bulk MoS2 into monolayer results in a considerable defect density that has extremely low photoluminescence quantum yield. It has been proposed that the electronic characteristic of the monolayer can be reproduced experimentally in MoS2 by K intercalation [1]. In this work, the effects of alkali metal intercalation (such as Li, Na, K, and Rb) are investigated by using first-principles calculations. The results show significant expansion of interlayer spacing and contribution of electron donation from alkali metal to the conduction band of MoS2. The expansion of the interlayer spacing depends on atomic radii of the intercalated metals. Moreover, band gap type is changed from indirect to direct because of the expansion of the interlayer spacing reduces the electronic interactions between adjacent layers creating a quasi-monolayer character. The effects of K concentration have been investigated by varying the number of K atoms in the 2×2×1 supercell of MoS2Kx (where x = 0.25, 0.50, 0.75 and 1.00). In order to compare the results from supercell calculations with the primitive cell, the electronic structures from supercell calculations are unfolded [2] onto the high symmetry paths as defined in the first Brillouin zone of the primitive cell. It has been found that the interlayer spacing of MoS2K0.25 is large enough to exhibit quasi-monolayer character. The unfolded electronic structures show a direct band gap located at the K point with larger band gap than bulk MoS2. The Fermi level is a bit higher than conduction band minimum due to electron donation from alkali metal. Our results suggest that different atomic radii and concentration of intercalated alkali metals could provide an opportunity to tune band gap type and value of TMDC materials.
[1] Eknapakul, T., et al., Electronic structure of a quasi-freestanding MoS(2) monolayer. Nano Lett, 2014. 14(3): p. 1312-6.
[2] Tomić, M., H.O. Jeschke, and R. Valentí, Unfolding of electronic structure through induced representations of space groups: Application to Fe-based superconductors. Physical Review B, 2014. 90(19).
