High degree of circular polarization in WS2 spiral nanostructures induced by broken symmetry

Abstract

We present helicity resolved photoluminescence (PL) measurements of WS 2 spiral (SPI) nanostructures. We show that very high degree of circular polarization (DCP) (~94 ± 4%) is obtained from multilayer SPI samples at room temperature upon excitation with a circularly polarized laser at a wavelength near-resonant with the A-exciton (633 nm). TEM analysis showed that these SPI nanostructures have AB stacking in which the inversion symmetry is broken, and hence this leads to very high DCP. Comparison with PL from monolayer and bi-layer WS 2 samples, along with polarization resolved PL studies provide evidence for suppression of interlayer/intravalley scattering in the multilayer SPI samples.

Spin‐Polarized Photoluminescence in Au25(SC8H9)18 Monolayer‐Protected Clusters

“…The maximum DOCP amplitude recorded was 20% and was obtained from the integrated intensities of the 1.94 eV emission component at 4.5 K. By comparison, DOCP amplitudes reported for CdSe/CdS nanocrystals and WS 2 spiral nanostructures were approximately 50% and 96%, respectively. [ 26,27 ] These values suggest the PL was a super‐position of emission components, with one dominant state determining the DOCP. At 1.78 and 1.94 eV, the decrease in DOCP amplitude with increasing temperature was best modelled by a single exponential decay.…”

Giant Valley Coherence at Room Temperature in 3R WS 2 with Broken Inversion Symmetry

“…The interband transitions of the K (K ′ ) valley in both upper and lower layers exclusively couple to σ -(σ + ) circularly polarized light. Thus, 3R-stacked TMDCs preserve the same valley-contrasting Berry curvature and physical properties as the case of monolayer, demonstrated recently in 3R MoS 2 [13] and WS 2 spiral nanostructures [27] where the inversion symmetry is broken. Accordingly, 3R-stacked few-layer TMDCs provide an unprecedented candidate for valleytronics and quantum logics [13,[28][29][30].…”

Chiral Coupling of Valley Excitons and Light through Photonic Spin–Orbit Interactions

“…In contrast to the centrosymmetric AA′ stacking in natural TMDCs crystals, the AB stacking structure possesses broken inversion symmetry, which, as discussed above, is the key factor of a high valley polarization contrast. Barman et al have shown that ≈94 ± 4% of valley polarization contrast can be achieved in WS 2 spiral nanostructure at RT, which is a new record of TMDCs valley polarization contrast observed at RT99 and is much higher than that of bilayer TMDCs (Figure 3d). This discovery widely extends the options of TMDCs nanostructures to be applied in novel valleytronics devices.…”

“…c) Schematic of AA′ and AB stacking of WS 2 . Reproduced with permission 99. Copyright 2019, Springer Nature.…”

“…Note that spiral nanostructured WS 2 have higher degree of valley polarization than bilayer and monolayer WS 2 , although the intensity of PL decreases with the increase of the number of TMDCs layers. Reproduced with permission 99. Copyright 2019, Springer Nature.…”