Highly valley-polarized singlet and triplet interlayer excitons in van der Waals heterostructure

Abstract

Two-dimensional semiconductors feature valleytronics phenomena due to locking of the spin and momentum valley of the electrons. However, the valley polarization is intrinsically limited in monolayer crystals by the fast intervalley electron-hole exchange. Hetero-bilayer crystals have been shown to have a longer exciton lifetime and valley depolarization time. But the reported valley polarization was low; the valley selection rules and mechanisms of valley depolarization remains controversial. Here, we report singlet and brightened triplet interlayer excitons both with over 80% valley polarizations, cross-and co-polarized with the pump laser, respectively.This is achieved in WSe 2 /MoSe 2 hetero-bilayers with precise momentum valley alignment and narrow emission linewidth. The high valley polarizations allow us to identify the band minima in a hetero-structure and confirm unambiguously the direct band-gap exciton transition, ultrafast charge separation, strongly suppressed valley depolarization. Our results pave the way for using semiconductor heterobilayers to control valley selection rules for valleytronic applications. *

Twist-angle dependence of moiré excitons in WS2/MoSe2 heterobilayers

“…The devices used in this work are WS 2 /MoSe 2 heterobilayers with different twist angles θ , capped by few-layer hexagonal boron nitride (hBN). Details of sample fabrication and calibration of θ have been described elsewhere 20 , 21 and provided in the “Method” section. Figure 1 a shows the optical microscope image of a heterobilayer, where the sharp edges of two monolayers are aligned.…”

“…Figure 1 a shows the optical microscope image of a heterobilayer, where the sharp edges of two monolayers are aligned. The twist angle is θ = 59.8° ± 0.3°, determined optically using polarization-dependent second-harmonic-generation measurements 20 , 21 (see Supplementary Figs. 1 and 2 and Supplementary Notes 3 and 4 for details).…”

Giant Valley-Zeeman Splitting from Spin-Singlet and Spin-Triplet Interlayer Excitons in WSe2/MoSe2 Heterostructure

“…This high valley polarization arises from the long valley lifetime of the hole [51][52][53] . The optically excited electron-hole pairs can quickly dissociate and separate into two layers [66][67][68][69] , but the intervalley scattering of the hole is strongly inhibited 34,50,51 , especially for resonance excitation [35][36][37][38][39][40] . This sensitive temperature dependence of the valley polarization for both interlayer exciton can be easily illustrated in Fig.…”

Formation and tuning of moiré excitons in large-twist angle WS2/MoSe2 heterobilayers

“…The devices used in this work are WS 2 /MoSe 2 heterobilayers with different twist angles θ, capped by few-layer hexagonal boron nitride (hBN). Details of sample fabrication and calibration of θ have been described elsewhere [20,21] and provided in the Method section. Fig.…”

“…1a shows the optical microscope image of a heterobilayer, where the sharp edges of two monolayers are aligned. The twist angle is θ = 59.8 • ± 0.3 • , determined optically using polarization-dependent second-harmonic-generation measurements [20,21] (See Supplemental Material Fig.S1 and Fig.S2 for details).…”