Directional interlayer spin-valley transfer in two-dimensional heterostructures

Abstract

Van der Waals heterostructures formed by two different monolayer semiconductors have emerged as a promising platform for new optoelectronic and spin/valleytronic applications. In addition to its atomically thin nature, a two-dimensional semiconductor heterostructure is distinct from its three-dimensional counterparts due to the unique coupled spin-valley physics of its constituent monolayers. Here, we report the direct observation that an optically generated spin-valley polarization in one monolayer can be transferred between layers of a two-dimensional MoSe2–WSe2 heterostructure. Using non-degenerate optical circular dichroism spectroscopy, we show that charge transfer between two monolayers conserves spin-valley polarization and is only weakly dependent on the twist angle between layers. Our work points to a new spin-valley pumping scheme in nanoscale devices, provides a fundamental understanding of spin-valley transfer across the two-dimensional interface, and shows the potential use of two-dimensional semiconductors as a spin-valley generator in two-dimensional spin/valleytronic devices for storing and processing information.

Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

“…We would like to mention that under the excitation with circular polarized light, the high-energy peak IX ▼ is weakly co-polarized of about 20% (for Tbath = 3 K , P = 140 µW), whereas the low-energy peak IX  is unpolarized within the given noise level. Generally, the valley-and spin-polarization can be assumed to be preserved for charge carriers transferred across heterojunctions [27]. Therefore, we assign the observed polarization to the fact that our excitation energy of Elaser = 1.94 eV is in resonance with the (intralayer) B exciton transition in MoSe2 at higher temperatures and slightly below that value at low temperatures resulting in the generation of valley-polarized charge carriers in MoSe2.…”

“…Moreover, the exciton Bohr radius of only a few lattice sites is expected to enable the creation of rather dense exciton ensembles -a rigorous requirement for studying the quantum phase diagram of interacting exciton ensembles [1]. Moreover, van der Waals heterostructures feature a type-II band alignment [16,19,25,46], and therefore, they render an efficient transfer of photogenerated charge carriers such that electrons accumulate in one layer and holes in the other layer [23,27,47]. In turn, thermalized IXs are expected with a significantly enhanced IX lifetime and density [21,22,26].…”

“…In turn, thermalized IXs are expected with a significantly enhanced IX lifetime and density [21,22,26]. Furthermore, by optical spin-valley pumping in one layer, the spin-and valleydegrees of freedom are preserved by the interlayer transfer of polarized charge carriers [25,27,28,48]. While intralayer excitons in TMD monolayers are perfect in-plane dipoles fostering Förster-type energy transfer between hetero-bilayers [49], interlayer excitons possess a permanent out of plain component altering the coupling to light and enabling to tune the exciton properties by static electric fields perpendicular to the layers.…”

“…IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electronhole wave functions resulting in dense IX ensembles that are thermalized to the lattice temperature. Besides IX ensembles in III-V HS [5][6][7][8][9][10][11][12][13][14][15], hetero-bilayers prepared from semiconducting transition metal dichalcogenides (TMDs) exhibit superior potential for studying interacting IX ensembles [3,[16][17][18][19][20][21][22][23][24][25] with intriguing spin-and valley-properties [26][27][28]. At the same time, TMDs are truly two-dimensional (2D) crystals coupled to each other or to substrates by van der Waals forces.…”

Recent Advances in Quantum Effects of 2D Materials

“…Figure a shows a typical monolayer WSe 2 –MoSe 2 vdW heterostructure . Due to the large Coulomb interaction in 2D materials, and the prediction of staggered (type‐II) band alignment between the proper pair of TMDs, the interlayer excitons/trions can be generated in heterostructures building from different types of 2D semiconducting TMDs, as illustrated in Figure b . The depletion region is absent in these kinds of heterostructure due to the 2D nature, and the band offsets are large enough, thus ensuring a fast interlayer charge transfer for interlayer excitons formation (Figure c) .…”