The optical response of artificially twisted MoS$$_2$$ bilayers

Abstract

Two-dimensional layered materials offer the possibility to create artificial vertically stacked structures possessing an additional degree of freedom—theinterlayertwist. We present a comprehensive optical study of artificially stacked bilayers (BLs) MoS$$_2$$ 2 encapsulated in hexagonal BN with interlayer twist angle ranging from 0$$^{\circ }$$ ∘ to 60$$^{\circ }$$ ∘ using Raman scattering and photoluminescence spectroscopies. It is found that the strength of the interlayer coupling in the studied BLs can be estimated using the energy dependence of indirect emission versus the A$$_\text {1g}$$ 1g –E$$_\text {2g}^1$$ 2g 1 energy separation. Due to the hybridization of electronic states in the valence band, the emission line related to the interlayer exciton is apparent in both the natural (2H) and artificial (62$$^\circ $$ ∘ ) MoS$$_2$$ 2 BLs, while it is absent in the structures with other twist angles. The interlayer coupling energy is estimated to be of about 50 meV. The effect of temperature on energies and intensities of the direct and indirect emission lines in MoS$$_2$$ 2 BLs is also quantified.

Electrically Tunable Localized versus Delocalized Intralayer Moiré Excitons and Trions in a Twisted MoS$_2$ Bilayer

“…These results are in good agreement with the theoretical calculations of optical conductivity spectra of tBLs, 22,23 and ascertain the formation of exciton sub-bands in mSL of tBLs, enabled by small θ. The near-0 0 H-type stacking of tBL in D1 can be confirmed from the similar peak positions of both A 1g and E 1 2g Raman modes 26 dividual spectra (see Fig. S4).…”

Twist Angle-Dependent Interlayer Exciton in MoS2 Bilayers Revealed by Room-Temperature Reflectance

“…This means that, in our 2H sample, the strong interlayer coupling in the valence band is turned on and the A-B splitting is determined by the band offset and the interlayer hopping [26]. The slight interlayer coupling between the bottom layer valence band and the top layer conduction band is a symmetry allowed by small value, as the larger band is offset [24]. Thus, the typical observed A-B splitting is more or less the same as the monolayer.…”

“…The previous studies of the IE in bilayer MoS 2 have focused on the H or R stacking at cryogenic temperatures [17,19]. In the hBN encapsulated artificially stacked twist-angle bilayers, the IE PL emission was observed at low temperatures that appear only at the 62 • sample [24]. This IE emission was not clearly detectable at the evaluated temperature, as the relaxations to lower intralayer emissions or the non-radiative channels are active.…”

Temperature induced modulation of resonant Raman scattering in bilayer 2H-MoS$_{2}$

“…The low temperature (T =5 K) PL spectrum comprises of three distinct emission lines. They are ascribed to the negatively charged (X − ), neutral (X A ), and interlayer excitons (IL) formed in the vicinity of the A exciton 34,[48][49][50] . It can be seen that the temperature increase from 5 K to 300 K leads to the red shift of their emission energies, accompanied by the reduction in the corresponding intensities.…”

“…[18][19][20][21][22][23][24] In particular, the RS spectrum of the bilayer (BL) MoS 2 , which is one of the simplest prototypes of a multilayer van der Waals structure, uncovers several intriguing features. [25][26][27][28][29][30][31][32][33][34] The RS spectrum becomes especially rich under resonant excitation conditions. The resonant RS was extensively used to study the characteristics of phonon modes in MoS 2 .…”