Electrically Tunable Valley Dynamics in Twisted ${\mathrm{WSe}}_{2}/{\mathrm{WSe}}_{2}$ Bilayers

Electrically Tunable Valley Dynamics in Twisted ${WSe}_{2} / {WSe}_{2}$ Bilayers

Giovanni Scuri, Trond I. Andersen, You Zhou, Dominik S. Wild, Jiho Sung, Ryan J. Gelly, Damien Bérubé, Hoseok Heo, Linbo Shao, Andrew Y. Joe, Andrés M. Mier Valdivia, Takashi Taniguchi, Kenji Watanabe, Marko Lončar, Philip Kim, Mikhail D. Lukin, and Hongkun Park

Phys. Rev. Lett. 124, 217403 – Published 28 May 2020

Abstract

The twist degree of freedom provides a powerful new tool for engineering the electrical and optical properties of van der Waals heterostructures. Here, we show that the twist angle can be used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers. Specifically, we observe that the interlayer excitons in twisted ${WSe}_{2} / {WSe}_{2}$ bilayers exhibit a high ( $> 60 %$ ) degree of circular polarization (DOCP) and long valley lifetimes ( $> 40 ns$ ) at zero electric and magnetic fields. The valley lifetime can be tuned by more than 3 orders of magnitude via electrostatic doping, enabling switching of the DOCP from $\sim 80 %$ in the $n$ -doped regime to $< 5 %$ in the $p$ -doped regime. These results open up new avenues for tunable chiral light-matter interactions, enabling novel device schemes that exploit the valley degree of freedom.

Received 14 December 2019
Accepted 6 May 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.217403

Physics Subject Headings (PhySH)

Excitons Valleytronics

2-dimensional systems Transition metal dichalcogenides

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Giovanni Scuri ^1,*, Trond I. Andersen^1,*, You Zhou ^1,2,*, Dominik S. Wild¹, Jiho Sung^1,2, Ryan J. Gelly¹, Damien Bérubé³, Hoseok Heo^1,2, Linbo Shao ⁴, Andrew Y. Joe ¹, Andrés M. Mier Valdivia⁴, Takashi Taniguchi⁵, Kenji Watanabe ⁵, Marko Lončar⁴, Philip Kim^1,4, Mikhail D. Lukin^1,†, and Hongkun Park^1,2,‡

¹Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
²Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA
³Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
⁴John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
⁵National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

^*These authors contributed equally to this work.
^†To whom correspondence should be addressed. hongkun_park@harvard.edu,
^‡To whom correspondence should be addressed. lukin@physics.harvard.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 124, Iss. 21 — 29 May 2020

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Band engineering through twisting. (a) Side view and Brillouin zone alignment of natural (2H-stacked, top) and twisted homobilayers (bottom). The $K$ and $K^{'}$ points are aligned in natural bilayers, but not in twisted structures. (b) Device schematic (top), illustration of a moiré pattern in a twisted ${WSe}_{2} / {WSe}_{2}$ bilayer (bottom left), and optical image of a device (bottom right). (c) Polarization-resolved PL spectra from bilayers with varying twist angles. All spectra are collected in the intrinsic regime. Solid (dashed) lines represent co- (cross-) polarized emission. $X_{0}$ and $X_{I}$ indicate intralayer and interlayer exciton emission, respectively. (d) Degree of circular polarization calculated from the PL spectra in (c). While the natural bilayer (red) exhibits almost zero interlayer DOCP, the twisted devices show DOCP as high as 60%.
Reuse & Permissions
Figure 2
Gate tunability of DOCP. (a) Out-of-plane electric field dependence of intra- and interlayer exciton PL from the 17°-twisted bilayer. While the intralayer excitons ( $X_{0}$ ) do not shift with the electric field, the interlayer excitons ( $X_{I}$ ) exhibit a Stark shift equivalent to an electron-hole separation of 0.37 nm. Opposite voltages are applied to the two gates (the hBN flakes have the same thickness) to apply an electric field while keeping the TMD intrinsic. (b) Electric field dependence of DOCP, showing that the DOCP is largely insensitive to the electric field. (c) Doping dependence of PL emission for intra- and interlayer excitons. The same voltages are applied to the two gates to dope the sample without applying an electric field. (d) Doping dependence of DOCP, showing switching from large ( $> 80 %$ ) values in the $n$ -doped regime to almost zero ( $< 5 %$ ) in the $p$ -doped regime.
Reuse & Permissions
Figure 3
Exciton and valley dynamics in twisted ${WSe}_{2}$ . (a) Time-resolved measurements of co- and cross-polarized photoluminescence (dark and light green markers, respectively) from the 17°-twisted bilayer in the intrinsic regime. Black dotted lines are biexponential fits convolved with the instrument response (Supplemental Sec. IV [43]). The extracted slow and fast timescales are $τ_{1} = 0.1 ns$ and $τ_{2} = 0.9 (1.0) ns$ for co- (cross-) polarized emission. (b),(c) The same as in (a), but at $V_{G} = - 8 V$ and $V_{G} = 8 V$ . Black dashed lines are fits based on the coupled linear model (Supplemental Sec. IV [43]). The extracted exciton and valley lifetimes are $τ_{1} = 0.1 ns$ and $τ_{v} = 30 ps$ at $V_{G} = - 8 V$ and $τ_{1} = 40 ps$ and $τ_{v} = 2.2 ns$ at $V_{G} = 8 V$ . (d) Time-resolved DOCP corresponding to measurements in (a)–(c). Black dotted and dashed curves are calculated from fits in (a)–(c). Inset: Enlarged version of the long tail in the intrinsic regime, fitted with the linear coupled model (dashed line). The extracted valley lifetime is $τ_{v} = 44 ns$ . Lines corresponding to $τ_{v} = 5 ns$ (light gray) and $τ_{v} = 10 ns$ (dark gray) are shown for comparison.
Reuse & Permissions
Figure 4
Depolarization mechanisms in natural and twisted ${WSe}_{2} / {WSe}_{2}$ bilayers. (a) Depolarization in natural bilayer ${WSe}_{2}$ occurs simply through two spin-flip transitions (curved arrows), causing low DOCP. Magenta (cyan) bands indicate spin up (down) for electrons. The thickness of the dashed lines qualitatively represents the relative dominance of the $σ_{+}$ and $σ_{-}$ recombination processes. Inset: Brillouin zone showing the location of the degenerate $K$ and $Q$ points of a natural bilayer as a whole. (b) Depolarization in twisted bilayer ${WSe}_{2}$ requires both a phonon-induced momentum kick (straight arrows) and a spin-flip transition (curved arrows) for both carriers, resulting in high DOCP and long $τ_{v}$ . (c),(d) Since positively (negatively) charged excitons already have holes (electrons) at both the $K$ and $K^{'}$ ( $Q$ and $Q^{'}$ ) points, only the electron (hole) must flip spin and shift in $k$ space.
Reuse & Permissions

Physical Review Letters

Electrically Tunable Valley Dynamics in Twisted WSe2/WSe2 Bilayers

Giovanni Scuri, Trond I. Andersen, You Zhou, Dominik S. Wild, Jiho Sung, Ryan J. Gelly, Damien Bérubé, Hoseok Heo, Linbo Shao, Andrew Y. Joe, Andrés M. Mier Valdivia, Takashi Taniguchi, Kenji Watanabe, Marko Lončar, Philip Kim, Mikhail D. Lukin, and Hongkun Park

Phys. Rev. Lett. 124, 217403 – Published 28 May 2020

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

Supplemental Material (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Electrically Tunable Valley Dynamics in Twisted ${WSe}_{2} / {WSe}_{2}$ Bilayers