Coupling and Interlayer Exciton in Twist‐Stacked WS₂ Bilayers

Abstract

and Hofstadter's butterfl y. [ 10,11 ] In vertical stacked MoS 2 /WS 2 heterostructure which was fabricated by transferring MoS 2 fl akes to WS 2 fl akes, a new photoluminescence (PL) peak emerged after annealing in vacuum which is dictated by charge transfer and band normalization between the WS 2 and MoS 2 layers. [ 12 ] Also, indirect band gap peak of 15° twisted MoS 2 grown by chemical vapor deposition (CVD) method has a smaller redshift due to the larger interlayer distance compared to the ones of AA and AB stacked MoS 2 . [ 13 ] Although WS 2 has a similar atomic structure to MoS 2 , very different properties have been shown, such as larger valence band splitting of WS 2 (0.41 eV) [ 14 ] than MoS 2 (0.16 eV). [ 1,15 ] Also, the degree of circular polarization of A exciton emission under near-resonant excitation is around 95% in WS 2 bilayer [ 16 ] in contrast to 10%−30% in MoS 2 bilayer. [ 17 ] In this paper, we demonstrate the observation of WS 2 bilayers and trilayers with various twist angles on quartz plates by CVD method. The twisted bilayers have much intensive PL compared to both monolayers and untwisted bilayers (AA and AB stackings) and the absence of indirect transition peak. This implies that the random twisted WS 2 bilayer possesses a quasidirect band gap behavior as a result of weakened coupling due to enlarged interlayer distance. Experimental ResultsOur samples were growth by CVD method at 1100 °C using the setup shown in Figure 1 a. A magnet was applied to push the sulfur source in the furnace (see details in the Experimental Section). Bilayer WS 2 with twist angles of 0°, 13°, 30°, 41°, 60°, and 83° were observed from one experiment and on the same substrate as shown in Figure 1 b-g. The twist angle is defi ned by rotation of upper layer related to the lower layer at counterclockwise direction. We can see that the smaller upper monolayer WS 2 triangle is twisted to different angles in related to the bigger lower monolayer WS 2 in each sample.Monolayer WS 2 is a direct band gap material, whereas WS 2 bilayers with AA or AB stacking are known to have an indirect band gap due to the interlayer electronic coupling. [ 18,19 ] However, as the symmetry between upper and lower layer is broken, the random twisted WS 2 bilayer samples are expected to have Interlayer electronic and mechanical couplings of transitional metal dichalcogenides due to Van der Waals force determine their band structure and Raman modes evolution, respectively. Twist-stacked WS 2 bilayers have been synthesized with twist angles of 0°, 13°, 30°, 41°, 60°, and 83° via chemicalvapor depositon, which allows us to study the coupling effect by Raman and photoluminescence spectroscopy and density function calculation. The photoluminescence property implies that these random-twisted WS 2 bilayers behave as quasi-direct bandgap material due to weakened interlayer coupling as a result of larger interlayer distances than the nontwisted 0° and 60° stacked WS 2 bilayers (with an indirect band gap). In addition, an additional small peak (A...

Valley depolarization in downconversion and upconversion emission of monolayer WS2 at room temperature

“…All the results presented here are of S1 sample and those of S2 sample are presented in Supplementary material. The Raman spectrum of monolayer WS 2 is shown in Figure 1b, and the Raman interval between out-of-plane A 1g mode and inplane E 1 2g mode is around 61.7 cm −1 , which is in accordance with previous reports [47,48]. The schematic of DC PL and GS is the ground state.…”

Space-confined and substrate-directed synthesis of transition-metal dichalcogenide nanostructures with tunable dimensionality

“…1i presents the Raman spectra of WS 2 NRs with two primary peaks corresponding to the in-plane E 1 2g and out-of-plane A 1g modes, respectively. Moreover, we found that the A 1g /E 1 2g ratio for multilayer WS 2 NR (~1.23) is higher than the A 1g /E 1 2g ratio of monolayer WS 2 NR (~1.1) and monolayer WS 2 NP (~0.34), which may result from the interlayer coupling effect in the neighboring layers [28,29]. This distinct Raman feature can be viewed as a probe to differentiate monolayer and multilayer WS 2 NRs.…”

Classical and quantum phases in hexagonal boron nitride‐combined van der Waals heterostructures

“…For example, twisted bilayer WS 2 flakes with various twist angles exhibited strong PL and quasi-direct bandgaps compared with their conventional AA or AB stacking bilayers, implying an enlarged interlayer distance and a weaker interlayer coupling. 123 The unprecedented magic angle of twisted bilayer graphene and the resulting superconductivity surprised researchers while h-BN is crucial to fabricate twisted bilayer graphene as a pick-up tool and a capping layer. Two graphene monolayers, stacked by a small twist angle, form a moiré pattern with a period related to the twist angle.…”

“…By tuning the rotational angle, interlayer coupling in Gr/Gr can be precisely controlled and employed to modulate their electronic and optical properties. For example, twisted bilayer WS 2 flakes with various twist angles exhibited strong PL and quasi‐direct bandgaps compared with their conventional AA or AB stacking bilayers, implying an enlarged interlayer distance and a weaker interlayer coupling 123 …”