Electronic Structure of Exfoliated Millimeter-Sized Monolayer WSe2 on Silicon Wafer

TL;DR

This study reports the fabrication of high-quality, large-area monolayer WSe2 crystals and their detailed electronic band structure analysis, revealing key properties relevant for nanoelectronics and valleytronics applications.

Contribution

We developed an improved exfoliation method to produce millimeter-sized monolayer WSe2 and performed comprehensive electronic structure measurements.

Findings

Monolayer WSe2 is a 1.2 eV direct band gap semiconductor.

Valence band spin-splitting at K point is 460 meV.

Substrate screening significantly affects electronic properties.

Abstract

The monolayer WSe2 is interesting and important for future application in nanoelectronics, spintronics and valleytronics devices, because it has the largest spin splitting and longest valley coherence time among all the known monolayer transition-metal dichalcogenides (TMDs). To obtain the large-area monolayer TMDs' crystal is the first step to manufacture scalable and high-performance electronic devices. In this letter, we have successfully fabricated millimeter-sized monolayer WSe2 single crystals with very high quality, based on our improved mechanical exfoliation method. With such superior samples, using standard high resolution angle-resolved photoemission spectroscopy, we did comprehensive electronic band structure measurements on our monolayer WSe2. The overall band features point it to be a 1.2eV direct band gap semiconductor. Its spin-splitting of the valence band at K point is found as 460 meV, which is 30 meV less than the corresponding band splitting in its bulk counterpart. The effective hole masses of valence bands are determined as 2.344 me at Gamma, and 0.529 me as well as 0.532 me at K for the upper and lower branch of splitting bands, respectively. And screening effect from substrate is shown to substantially impact on the electronic properties. Our results provide important insights into band structure engineering in monolayer TMDs. Our monolayer WSe2 crystals may constitute a valuable device platform.