Stacking-dependent exciton multiplicity in WSe2 bilayers

Abstract

Twisted layers of atomically thin two-dimensional materials realize a broad range of novel quantum materials with engineered optical and transport phenomena arising from spin and valley degrees of freedom and strong electron correlations in hybridized interlayer bands. Here, we report experimental and theoretical studies of WSe2 homobilayers obtained in two stable configurations of 2H (60° twist) and 3R (0° twist) stackings by controlled chemical vapor synthesis of high-quality large-area crystals. Using optical absorption and photoluminescence spectroscopy at cryogenic temperatures, we uncover marked differences in the optical characteristics of 2H and 3R bilayer WSe2 which we explain on the basis of beyond-DFT theoretical calculations. Our results highlight the role of layer stacking for the spectral multiplicity of momentum-direct intralayer exciton transitions in absorption, and relate the multiplicity of phonon sidebands in the photoluminescence to momentum-indirect excitons with different spin valley and layer character. Our comprehensive study generalizes to other layered homobilayer and heterobilayer semiconductor systems and highlights the role of crystal symmetry and stacking for interlayer hybrid states.

Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits

“…The peak observed at 308 cm –1 (labeled as B′ 2g ) is indicative of regions with a thickness of 2 monolayers. Based on the described Raman data, the intense B′ 2g peak confirms the bilayer behavior of the deposed WSe 2 , , and the splitting of the two main out-of- and in-plane modes indicates a type of stacking that can be attributed to the 3R phase. Additional micro-Raman spectra, obtained from different regions of the heterostructures with varying distinct numbers of stacked van der Waals materials, are provided in Section 1 in the Supporting Information.…”

Quasi van der Waals Epitaxy of Rhombohedral-Stacked Bilayer WSe2 on GaP(111) Heterostructure

“…The line width of the A 1g and E′ 2g peaks is 2.5 cm –1 , calculated by the full width at half-maximum (fwhm) after a Lorentzian fitting, which indicate that our MBE 3R WSe 2 /GaP­(111)B bilayer has a slightly larger defect density than the reference CVD 3R WSe 2 crystals (1 cm –1 fwhm). The relative intensities of the peaks E′ 2g and A 1g may be also used as a measure of the 3R/2H stacking order . In the current case, this suggests that our sample is typically composed of the 3R phase.…”

“…To confirm the 3R character of our layer, we compare our MBE-grown WSe 2 (black curve) to reference CVD-grown 2H and 3R WSe 2 (blue and red curves, respectively) and to a 3R WSe 2 CVD-grown flake transferred over a graphene substrate (green curve). Using these reference CVD crystals, for which the 3R or 2H character has been unambiguously determined, , we observe that the A 1g mode is dominant over the E′ 2g frequency in the the case of the 2H WSe 2 bilayer (blue curve). However, in the 3R configuration (red and green curves), each single E′ 2g and A 1g mode can be resolved .…”

Imaging lattice reconstruction in homobilayers and heterobilayers of transition metal dichalcogenides

“…For CVD-grown WSe 2 homobilayers and WSe 2 -MoSe 2 heterostructures, our imaging technique revealed complete reconstruction into domains of a single registry, with mutually exclusive R M h and R X h stacking configurations in R-type flakes and the tendency of R M h domain predominance. As the role of complete lattice reconstruction has been neglected in previous studies of CVD-grown R-type WSe 2 [45], MoS 2 [46] and WS 2 [47] homobilayers, we anticipate our work to motivate theoretical studies of optoelectronic properties in these contrasting crystal realizations. Moreover, our findings have immediate consequences for the optical properties of heterobilayer stacks of semiconducting TMDs [11,44], and can be generalized to the entire class of van der Waals heterostructures.…”

“…After the growth, the furnace was opened for a rapid cool-down to room temperature. Homobilayer WSe 2 crystals were synthesized on SiO 2 /Si substrates (with 285 nm SiO 2 ) with the vapor phase chalcogenization technique using ultra-pure WO 2 and Se powders from Sigma-Aldrich as precursors and process details described in our previous work [45]. WSe 2 -MoSe 2 heterobilayers were grown using a two-step CVD process, as described in our previous work [44].…”