Twisted Multilayer MoS2 Growth on SrTiO3 for 2D materials

Monolayer two dimensional (2D) transition metal dichalcogenides (TMDs) are recent emerged one among all other optoelectronic material, because of its strong light−matter interactions for direct bandgap. However, these multilayer counter parts exhibited an indirect bandgap resulting to poor quantum yield of photon emission. Sinu Mathew and his collaborators have been reported on the strong direct bandgap-like photoluminescence (PL) at ∼1.9 eV, a multilayer MoS₂ grown on SrTiO₃ in ACS Nano on November 2020. They found that the intensity is significantly higher than the observed multilayer MoS₂/SiO₂. However, it affects the evolution of band structure in multilayer MoS₂ which causes the higher photo carrier recombination for the direct bandgap. To enable multilayer TMDs for robust optical device applications, these results will provide a suitable platform. In this research, they have demonstrated a strong direct bandgap-like PL emission via chemical vapor deposition (CVD) growth multilayer MoS₂ on SrTiO₃ (STO), with an archetypal perovskite transition metal oxide substrate [1].

2D transition materials are characterized of weak out-of-plane van der Waal (vdW) forces even without lattice or interlayer crystallographic orientation matching which can easily stack them to the formation of vertical heterostructures. It can be seen that the 2D materials families will have the monolayers of TMD semiconductors, which can exhibit several intriguing properties like strong direct bandgap photoluminescence (PL). MoS₂have been popular in the visible range with tightly bound excitons and trions [2].

In 2D materials, TMDs has attracted an significant interest on the evolution of interlayer coupling and its dramatic effects on band response in structural features. Meanwhile, the monolayers have direct bandgap, bilayer and multilayer TMDs are indirect bandgap, which results to a very weak PL emission. It can also be seen that the multilayers have exhibited indirect bandgap and low PL yield, the increase of optical density with the thickness is a challenging one and for the enhancement of this yield would be required modification in the band structure. However, there are some effort in this direction, that led to two major approaches [3]:

Intercalation of light atomic species in the van der Waals gap.
Fabrication of bilayer heterostructures with an interlayer twist.

As per supportive evidence, the MoS₂ domains are randomly oriented and their relative orientation layers would be different during the coalesce of continuous film formation. Hence, this results to an interlayer twist, which could cause the multilayer MoS₂ film deviation from conventional 2H configuration. Notably, to illustrate the strong luminescence from the twisted multilayer samples, an interlayer twist will influence PL intensity, and the changes observed are not adequate. For instance, the interlayer twist in bilayer MoS₂/SiO₂will not considerably enhance the PL emission intensity. Meanwhile for twisted bilayer MoS₂/STO, the change in PL intensity between a twisted bilayer and a monolayer sample would be negligible (almost 0.7% of the monolayer).

Figure 1. Schematic diagram, the crystal structure of a 2-H oriented bilayer MoS₂compared with the crystal structure of twisted MoS₂/STO [1].

An observation of strong direct bandgap-like PL from multilayer MoS₂ grown on STO at 1.9 eV is mainly due to the combined effect of interlayer twist and enhanced vdW gap. The possible reason for the enhanced vdW gap separation on the samples is mainly due to growth of multilayer MoS₂ exposure at elevated temperatures for a prolonged period of time. This will cause oxygen out diffusion from the STO lattice. As per the previous reports, similar oxygen out diffusion was observed in oxide thin film heterostructures. Therefore, in their results they predict that the oxygen in their case might be intercalated between the MoS₂ which lead to further expansion of the vdW gap resulting to an enhanced interlayer decoupling [4,5].

One of the main key differences in the growth of multilayer MoS₂ on STO is to compared their growth on SiO₂which induced interlayer twist and mild self-intercalation of oxygen, the oxygen in STO is highly mobile at a growth temperature of ∼700 °C. Hence, this weakens the interlayer coupling, which leads to a strong light emission. It can also be an interesting fact that the exploration the multilayer growth TMDs on complex oxide perovskites similar to that of SrTiO₃ such as BaTiO₃ and EuTiO₃, which exhibit structural phase transitions, spontaneous charge ordering.

Our SNB team have emphasize this research article to enrich our viewer’s knowledge about the twisted multilayer MoS₂ growth on SrTiO₃ for a strong photoluminescence. Observation of strong PL emission from multilayer MoS₂ samples can enable its use for the applications. This require a strong PL emission and absorption such as nanoscale detectors and light emitters where they used only the monolayers. Being a single layer of atoms, monolayer MoS₂and other related TMDs are sensitive to ambient, that degrades its performance in over time. Thus, the observation of strong PL emission from multilayered MoS₂ could offer a better solution which would be more stable in the performance. Therefore, a fundamental perspective, like mixed dimensional heterostructures can provide additional pathways to achieve optoelectronic tunability in 2D semiconductors.

References

S. Sarkar et al., ACS Nano (2020),https://dx.doi.org/10.1021/acsnano.0c04801.
G. Wang, et al., Rev. Mod. Phys. 90, 021001 (2018).
R.Dhall,et al.,Adv. Mater., 27, 1573 (2015).
De Souza, Adv. Funct. Mater., 25, 6326 (2015).
L.Hu, et al.,Appl. Phys. Lett. 105, 111607 (2014).

--- Dr. Y. Sasikumar

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