[Paper Review] Observation of Ultralong Valley Lifetime in WSe2/MoS2 Heterostructures

Jonghwan Kim, Chenhao Jin+10 more|arXiv (Cornell University)|Dec 16, 2016

2D Materials and Applications30 references20 citations

TL;DR

This study demonstrates ultralong valley lifetime in WSe2/MoS2 heterostructures by leveraging ultrafast charge transfer to create valley-polarized holes in WSe2. The holes exhibit near-unity valley polarization with a depolarization lifetime exceeding 40 microseconds at 10 K—orders of magnitude longer than previous reports—enabling new prospects for valleytronics and spintronics applications.

ABSTRACT

The valley degree of freedom in two-dimensional (2D) crystals recently emerged as a novel information carrier in addition to spin and charge. The intrinsic valley lifetime in 2D transition metal dichalcoginides (TMD) is expected to be remarkably long due to the unique spin-valley locking behavior, where the inter-valley scattering of electron requires simultaneously a large momentum transfer to the opposite valley and a flip of the electron spin. The experimentally observed valley lifetime in 2D TMDs, however, has been limited to tens of nanoseconds so far. Here we report efficient generation of microsecond-long lived valley polarization in WSe2/MoS2 heterostructures by exploiting the ultrafast charge transfer processes in the heterostructure that efficiently creates resident holes in the WSe2 layer. These valley-polarized holes exhibit near unity valley polarization and ultralong valley lifetime: we observe a valley-polarized hole population lifetime of over 1 us, and a valley depolarization lifetime (i.e. inter-valley scattering lifetime) over 40 us at 10 Kelvin. The near-perfect generation of valley-polarized holes in TMD heterostructures with ultralong valley lifetime, orders of magnitude longer than previous results, opens up new opportunities for novel valleytronics and spintronics applications.

Motivation & Objective

• To achieve long-lived valley polarization in 2D transition metal dichalcogenides (TMDs) for practical valleytronics applications.
• To overcome the limitation of short valley lifetimes (typically tens of nanoseconds) observed in previous TMD experiments.
• To exploit interlayer charge transfer in WSe2/MoS2 heterostructures to generate and stabilize valley-polarized holes.
• To measure and demonstrate valley depolarization lifetimes significantly exceeding those reported in monolayer TMDs.
• To establish a platform for scalable, room-temperature-compatible valleytronic devices using heterostructured 2D materials.

Proposed method

• Utilized a WSe2/MoS2 van der Waals heterostructure to enable ultrafast interlayer charge transfer (occurring in <100 fs).
• Optically excited the MoS2 layer to generate electron-hole pairs, with holes rapidly transferred to the WSe2 layer.
• Employed time-resolved pump-probe spectroscopy to monitor valley polarization dynamics in the WSe2 layer.
• Measured valley polarization using circularly polarized light to selectively excite K+ or K− valleys in WSe2.
• Conducted experiments at 10 K to minimize thermal scattering and enhance valley lifetime measurements.
• Analyzed the decay of valley polarization to extract the valley depolarization lifetime from the temporal evolution of circularly polarized luminescence.

Experimental results

Research questions

• RQ1 Can valley polarization be stabilized in 2D heterostructures with lifetimes significantly longer than tens of nanoseconds?
• RQ2 What role does ultrafast interlayer charge transfer play in generating long-lived valley-polarized holes in WSe2?
• RQ3 What is the maximum achievable valley depolarization lifetime in WSe2/MoS2 heterostructures at low temperature?
• RQ4 To what extent can valley polarization be preserved in WSe2 due to spin-valley locking and suppressed inter-valley scattering?
• RQ5 Can near-unity valley polarization be achieved in a 2D heterostructure system with minimal non-radiative decay pathways?

Key findings

• A valley-polarized hole population lifetime exceeding 1 microsecond was observed in WSe2/MoS2 heterostructures at 10 K.
• The valley depolarization lifetime—defined as the inter-valley scattering time—was measured to be over 40 microseconds at 10 K.
• Near-unity valley polarization (close to 100%) was achieved in the WSe2 layer due to efficient and selective charge transfer from MoS2.
• The ultralong valley lifetime is attributed to strong spin-valley locking and suppressed intervalley scattering, enhanced by the heterostructure's band alignment.
• Ultrafast charge transfer (occurring in less than 100 fs) efficiently populates the WSe2 layer with valley-polarized holes, minimizing recombination losses.
• The results represent an improvement of more than two orders of magnitude in valley lifetime compared to previous reports in monolayer TMDs.