Local Spectroscopic Characterization of Spin and Layer Polarization in WSe$_2$

TL;DR

This study uses STM and STS to analyze the electronic, spin, and layer properties of monolayer and bilayer WSe2, revealing large band gaps, spin-splitting, and spin-valley-layer coupling.

Contribution

It provides the first detailed spectroscopic mapping of spin and layer polarization in WSe2 using Fourier transform STS techniques.

Findings

Measured a 2.21 eV band gap in monolayer WSe2.

Identified large spin-splitting in the valence band.

Observed spin-valley-layer coupling in bilayer WSe2.

Abstract

We report scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of monolayer and bilayer WSe$_2$. We measure a band gap of 2.21 $\pm$ 0.08 eV in monolayer WSe$_2$, which is much larger than the energy of the photoluminescence peak indicating a large excitonic binding energy. We additionally observe significant electronic scattering arising from atomic-scale defects. Using Fourier transform STS (FT-STS), we map the energy versus momentum dispersion relations for monolayer and bilayer WSe$_2$. Further, by tracking allowed and forbidden scattering channels as a function of energy we infer the spin texture of both the conduction and valence bands. We observe a large spin-splitting of the valence band due to strong spin-orbit coupling, and additionally observe spin-valley-layer coupling in the conduction band of bilayer WSe$_2$.