Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides

TL;DR

This paper demonstrates a universal method for tuning the bandgap of 2D transition-metal dichalcogenides via surface doping, revealing a controllable range from visible to near-infrared and elucidating the underlying surface Stark effect mechanism.

Contribution

It introduces a universal surface doping technique to modulate the bandgap in TMDs, supported by spectroscopic evidence of symmetry breaking and spin splitting.

Findings

Bandgap can be tuned from 0.8 to 2.0 eV across TMDs.

Surface Stark effect is identified as the universal mechanism.

Bandgap transition from indirect to direct observed.

Abstract

Two-dimensional (2D) van-der-Waals semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic bandgap. For realistic applications, the ideal is to modify the bandgap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable bandgap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the bandgap of TMDs at the zone corners is modulated in the range of 0.8 ~ 2.0 eV, which covers a wide spectral range from visible to near infrared, with a tendency from indirect to direct bandgap. A key clue to understand the mechanism of this bandgap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of bandgap engineering based on the strong 2D nature of van-der-Waals semiconductors.