Valley lifetimes of conduction band electrons in monolayer WSe2

One of the main tasks in the investigation of 2-dimensional transition metal dichalcogenides is the determination of valley lifetimes. In this work, we combine time-resolved Kerr rotation with electrical transport measurements to explore the gate-dependent valley lifetimes of free conduction band electrons of monolayer WSe2. When tuning the Fermi energy into the conduction band we observe a strong decrease of the respective valley lifetimes which is consistent with both spin-orbit and electron-phonon scattering. We explain the formation of a valley polarization by the scattering of optically excited valley polarized bright trions into dark states by intervalley scattering. Furthermore, we show that the conventional time-resolved Kerr rotation measurement scheme has to be modified to account for photo-induced gate screening effects. Disregarding this adaptation can lead to erroneous conclusions drawn from gate-dependent optical measurements and can completely mask the true gatedependent valley dynamics.

[1]  How Photoinduced Gate Screening and Leakage Currents Dynamically Change the Fermi Level in 2D Materials , 2020, physica status solidi (RRL) – Rapid Research Letters.

[2]  Valley excitons in two-dimensional semiconductors , 2015, 1507.08103.

[3]  C. Stampfer,et al.  Spatial Control of Laser-Induced Doping Profiles in Graphene on Hexagonal Boron Nitride. , 2015, ACS applied materials & interfaces.

[4]  Wang Yao,et al.  Spin and pseudospins in layered transition metal dichalcogenides , 2014, Nature Physics.

[5]  A. Knorr,et al.  Dark excitons in transition metal dichalcogenides , 2017, 1709.00941.

[6]  S. Winnerl,et al.  Exciton localization in MoSe2monolayers induced by adsorbed gas molecules , 2019, Applied Physics Letters.

[7]  R. Rold'an,et al.  Spin-orbit-mediated spin relaxation in monolayer MoS2 , 2013, 1303.5860.

[8]  C. Robert,et al.  In-Plane Propagation of Light in Transition Metal Dichalcogenide Monolayers: Optical Selection Rules. , 2017, Physical review letters.

[9]  Xiaodong Xu,et al.  Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides , 2014, Nature Communications.

[10]  T. Taniguchi,et al.  Photoinduced doping in heterostructures of graphene and boron nitride. , 2014, Nature nanotechnology.

[11]  C. Stampfer,et al.  Intervalley dark trion states with spin lifetimes of 150 ns in WSe 2 , 2017, 1702.03712.

[12]  Wolfgang Albrecht,et al.  HNF - Helmholtz Nano Facility , 2017 .

[13]  Andrea Marini,et al.  Ab Initio Calculations of Ultrashort Carrier Dynamics in Two-Dimensional Materials: Valley Depolarization in Single-Layer WSe2. , 2017, Nano letters.

[14]  C. Stampfer,et al.  Unveiling valley lifetimes of free charge carriers in monolayer WSe2. , 2019, Nano letters.

[15]  Tianjiao Wang,et al.  Reversible photo-induced doping in WSe2 field effect transistors. , 2019, Nanoscale.